Document:

<PAGE>

                                                                   EXHIBIT 4.3

NEITHER THE SECURITY EVIDENCED BY THIS WARRANT NOR THE SECURITIES ISSUABLE UPON
EXERCISE OF THIS WARRANT HAVE BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933,
AS AMENDED, OR ANY APPLICABLE STATE LAW, AND NO INTEREST HEREIN OR THEREIN MAY
BE SOLD, DISTRIBUTED, ASSIGNED, OFFERED, PLEDGED OR OTHERWISE TRANSFERRED UNLESS
(A) THERE IS AN EFFECTIVE REGISTRATION STATEMENT UNDER SUCH ACT AND APPLICABLE
STATE SECURITIES LAWS COVERING ANY SUCH TRANSACTION INVOLVING SAID SECURITIES,
(B) THE COMPANY RECEIVES OPINION OF LEGAL COUNSEL FOR THE HOLDER OF SAID
SECURITIES (REASONABLY ACCEPTABLE TO THE COMPANY) STATING THAT SUCH TRANSACTION
IS EXEMPT FROM REGISTRATION, OR (C) THE COMPANY OTHERWISE SATISFIES ITSELF THAT
SUCH TRANSACTION IS EXEMPT FROM REGISTRATION.

No. ____

ISSUED: October 20, 1999

                               CRITICAL PATH, INC.

                          COMMON STOCK PURCHASE WARRANT

        THIS IS TO CERTIFY that, subject to the terms and conditions hereof,
U.S. Telesource, Inc., a Delaware corporation (the "Holder"), or its assigns is
entitled to subscribe for and purchase from Critical Path, Inc., a California
corporation (the "Company"), at any time on or after the date hereof but not
later than 5:00 p.m., San Francisco time, on October 19, 2007 (the "Exercise
Period"), subject to the provisions hereof, up to a maximum of 3,543,539 shares
(subject to adjustment as provided herein) (the "Warrant Shares" ) of fully paid
and non-assessable shares of Common Stock, $.001 par value, of the Company (the
"Common Stock"), at a price per share (the "Exercise Price") as set forth in
Section 1.1 hereto. This Warrant is being issued pursuant to a certain Master
Agreement dated as of October 20, 1999, between Qwest Communications
Corporation, a Delaware corporation, and the Company (the "Master Agreement").
All capitalized terms used but not otherwise defined herein shall have the
meaning ascribed to such terms in the Master Agreement.

                               SECTION 1 EXERCISE

        1.1 Warrant Shares Eligible to be Purchased

                (a) Subject to the provisions of Sections 1.2, 1.3, 1.4 and 12,
                the Holder's rights to purchase Warrant Shares during the
                Exercise Period shall vest and the Exercise Price for such
                Warrant Shares shall be determined as set forth in the table
                below. For the purposes of determining the vesting requirements,
                a "Qwest Service Email Box" shall mean an email box registered
                by Qwest utilizing Critical Path's Services, as provided in
                Article 3 of the Master Agreement, provided that all such Qwest
                Email Service Boxes shall be Critical Path sub-branded. For the
                purposes of determining the Exercise Price, the "Closing Price"
                shall mean the closing price of Company Common Stock, as

                                      -1-
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                average of the closing bid and asked prices of the Common Stock
                as quoted in the Over-the-Counter Market Summary or the last
                reported sale price of the Common Stock or the closing price
                quoted on the Nasdaq national market system, on the last trading
                day National Market System or on any exchange on which the
                Common Stock is then listed, whichever is applicable, for the
                fifteen (15) trading days prior to the day in question.

<TABLE>
<CAPTION>
Number of Shares                         Vesting                                Exercise Price
----------------                         -------                                --------------
<S>                                     <C>                                    <C>
589,090 shares, or one-sixth of          The registration of the                The Closing Price for the
the Warrant Shares.                      400,000th Qwest Service                Effective Date (the "Initial
                                         Email Box Upon execution of            Exercise Price") ($41.581)
                                         this Warrant.

589,090 shares, or one-sixth of          The registration of the                The Initial Exercise Price plus
the Warrant Shares.                      400,000th Qwest Service                $3.00 ($44.581)
                                         Email Box.

589,090 shares, or one-sixth of          The registration of the                The Initial Exercise Price plus
the Warrant Shares.                      800,000th Qwest Service                $6.00 ($47.581)
                                         Email Box.

589,090 shares, or one-sixth of          The registration of the                The Initial Exercise Price plus
the Warrant Shares.                      1,200,000th Qwest Service              $9.00 ($50.581)
                                         Email Box.

589,090 shares, or one-sixth of          The registration of the                The Initial Exercise Price plus
the Warrant Shares.                      1,600,000th Qwest Service              $12.00 ($53.581)
                                         Email Box.

589,090 shares, or one-sixth of          The registration of the                The Initial Exercise Price plus
the Warrant Shares.                      2,000,000th Qwest Service              $15.00 ($56.581)
                                         Email Box.
</TABLE>

                (b) Notwithstanding anything herein to the contrary, if any
                portion of the Warrant Shares shall not have vested (in
                accordance with the table set forth in Section 1(a) above)
                within three years from the date of the Master Agreement, then
                the Holder's right to purchase such portion of the Warrant
                Shares shall terminate.

                (c) Notwithstanding the foregoing, any portion of the Warrant
                Shares that have vested (in accordance with the table set forth
                in Section 1(a) above) shall be exercisable for a period of five
                years commencing on the date such portion of the Warrant Shares
                vested, and thereafter any such rights to exercise such portion
                of the Warrant Shares shall terminate.

        1.2 Procedure for Exercise

        Subject to the foregoing, this Warrant may be exercised by the Holder,
as to those shares of Warrant Shares for which this Warrant is then exercisable
as determined in accordance with Section 1.1, at any time during the Exercise
Period in whole or part by delivering to the Company, at the address of the
Company set forth in Section 17, (a) the form of Exercise Notice attached hereto
duly completed and executed by the Holder, (b) this Warrant certificate, (c)
cash or a bank cashier's check payable to the Company in the amount of the
Exercise Price multiplied by the number of shares for which this Warrant is
being exercised (the "Purchase Price"), and

                                      -2-
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(d), a certificate signed by an appropriate officer of the Holder setting forth
the amount of all outstanding securities, including rights, options, or warrants
to acquire securities of the Company, that are owned by the Holder, and any
Subsidiary (as defined in Section 16) of the Holder, in order to assure
compliance with Section 1.4 below. The Holder will be deemed to be the holder of
record of the shares of Common Stock as to which the Warrant was exercised in
accordance with this Warrant, effective at the close of business, San Francisco
time, on the date such exercise is completed and all documents specified above
are delivered to the Company.

        1.3 Net Exercise

        Notwithstanding the payment provisions set forth above, the Holder may
elect to exercise this Warrant by converting this Warrant into shares of Warrant
Shares as provided in this Section 1.3, such election to be effected by
surrender of this Warrant at the principal office of the Company, together with
the Notice of Exercise indicating such election, in which case the Company shall
issue to the Holder the number of shares of Warrant Shares determined as
follows:

                               X  = Y (A-B)
                                    -------
                                       A

Where:          X = the number of shares of Warrant Shares to be issued

                Y = the number of shares of Warrant Shares as to which the
                Warrant is being exercised

                A = the Fair Market Value (as defined below) of one (1) share of
                Warrant Shares

                B = the applicable Exercise Price

                For purposes of the above calculation, the Fair Market Value of
        a share of Warrant Shares shall be determined in good faith by the Board
        of Directors of the Company (the "Board"); provided, however, that if a
        public market for the common stock of Company (the "Common Stock")
        exists at the time of such exercise, then such Fair Market Value shall
        be the average of the closing bid and asked prices of the Common Stock
        as quoted in the Over-the-Counter Market Summary or the last reported
        sale price of the Common Stock or the closing price quoted on the Nasdaq
        National Market System or on any exchange on which the Common Stock is
        then listed, whichever is applicable, for the five (5) trading days
        prior to the date of exercise of this Warrant. The Board shall promptly
        respond in writing to an inquiry by the Holder as to the Fair Market
        Value of one share of Warrant Shares.

        1.4 Restrictions on Exercise

        This Warrant shall not be exercisable as to any portion of the Warrant
Shares that would cause, on the date of the vesting of each one-sixth of the
Warrant Shares (as described in Section 1.1) (each a "Vesting Date"), the Holder
and any Affiliate (as defined in the Master Agreement) thereof to own, in the
aggregate, 10% or more of the Fully-Diluted Stock (as defined in Section 1.5
below) of the Company. Subject to the next two succeeding sentences, if on the
Exercise Date such restriction is applicable, the Warrant shall be exercisable
for a number of Warrant Shares equal to the remainder of (x) the number of
shares equal to 9.9% of the Fully-Diluted Stock of the Company minus (y) the
number of shares of Common Stock then owned in the aggregate by the Holder and
any Affiliate (the "Maximum Number of Warrant Shares"). The Holder's right to
purchase any vested Warrant Shares in excess of the Maximum Number of

                                      -3-
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Warrant Shares will terminate on the date that is six months after the most
recent Vesting Date, except that with respect to the last Vesting Date, such
right will terminate thereon.

        1.5 Representation of Company Regarding the Equity Percentage of the
Warrants

        The Company represents to Holder that, as of the date of the Master
Agreement (the "Designated Date"), the Warrant Shares, if all issued as of the
Designated Date, would constitute six percent (6%) of the number of shares of
voting capital stock of the Company on the Designated Date after giving effect
to the exercise, exchange or conversion of all outstanding securities, rights,
options, warrants (including this Warrant), calls, commitments or agreements of
any nature or character (whether debt or equity) that are, directly or
indirectly, exercisable or exchangeable for, or convertible into or otherwise
represent the right to purchase or otherwise receive, directly or indirectly,
any such capital stock or other arrangement to acquire at any time or under any
circumstance, voting capital stock of the Company or any such other outstanding
securities and including all shares or other equity interests subject, as of the
Designated Date to issued stock options or other rights to acquire equity of any
nature to officers, directors, employees or consultants of the Company under all
agreements, plans or arrangements theretofore approved by the Board of Directors
of the Company, whether on not the right to exercise such outstanding options or
other rights is currently effective or vested (collectively, the "Fully-Diluted
Stock").

                     SECTION 2 DELIVERY OF STOCK CERTIFICATE

        Within 20 days after the exercise of this Warrant (in full or in part)
and payment of the Purchase Price then due, the Company at its expense shall
issue in the name of and deliver to the Holder (a) a certificate or certificates
for the number of fully paid and nonassessable shares of Warrant Shares to which
the Holder shall be entitled upon such exercise and (b) if applicable, a new
Warrant of like tenor to purchase up to that number of shares of Warrant Shares,
if any, as to which this Warrant shall not have been previously exercised by the
Holder or repurchased by the Company.

                    SECTION 3 COVENANTS AS TO WARRANT SHARES

        The Company covenants and agrees that the Company will at all times have
authorized and reserved a sufficient number of shares of Common Stock to provide
for the exercise of the rights represented by this Warrant. The Company further
covenants that all shares of Common Stock which may be issued upon the exercise
of the rights represented by this Warrant, will, upon issuance, in accordance
with the terms of this Warrant, be validly issued, fully paid and non-assessable
and free from all taxes, liens and charges solely with respect to the issuance
thereof. The Company further covenants and agrees that the Company will from
time to time take all such action as may be requisite to assure that the stated
or par value per share of the Common Stock is at all times equal to or less than
the then effective Exercise Price per share of the Common Stock issuable upon
exercise of this Warrant. If and so long as the Common Stock issuable upon
exercise of the rights represented by this Warrant is listed on any national
securities exchange, the Company will, if permitted by the rules of such
exchange, use its best efforts to list and keep listed on such exchange, upon
official notice of issuance, all Warrant Shares.

                                      -4-
<PAGE>

                       SECTION 4 EFFECTS OF REORGANIZATION

        In the event of a merger or consolidation of the Company with another
entity or the acquisition of all or substantially all of the assets or stock of
the Company by another entity (collectively such events being a
"Reorganization"), the following provisions will apply:

        (a) The Holder will be provided with notice of such Reorganization at
        the same time as notice is provided to the Company's shareholders. In
        addition, the Holder will be provided copies of any notice sent to
        shareholders of the Company in connection with such Reorganization
        (including notice of any shareholder's meetings and shareholder's
        consents), simultaneously with such notice being provided to any
        shareholder.

        (b) In the event the shareholders of the Company receive cash, stock or
        other property or contractual rights in respect of their stock in the
        Company (including upon any Reorganization where the Company is not the
        surviving entity), this Warrant will be exchanged for a warrant to
        purchase such kind and number of shares of capital stock or other
        securities or property or rights of the Company or the surviving entity
        to which the Holder would have been entitled if it had held the Common
        Stock issuable upon the exercise hereof immediately prior to such
        Reorganization, which warrant shall have the same terms and conditions
        hereof; provided, however, that if the Holder does not receive such
        warrant in exchange for this Warrant, then the Warrant will vest and
        become fully exercisable with respect to the maximum number of Warrant
        Shares upon completion of the Reorganization and the Holder will be
        entitled to exercise such Warrant Shares effective concurrently with
        completion of the Reorganization (but immediately prior thereto).
        Despite the foregoing, if the sole consideration received by
        shareholders in any such Reorganization is cash, then Company shall not
        be required to issue an exchange warrant if it pays to the Holder,
        immediately upon closing, cash based on the amount that the Holder would
        have received in such Reorganization upon full exercise of the Warrant
        for all Warrant Shares then vested; it being understood that the net
        amount payable will be the net exercise price therefor calculated in the
        manner specified in Section 1.3 above.

           SECTION 5 ADJUSTMENTS FOR STOCK SPLITS AND SIMILAR MATTERS

        5.1 Stock Splits and Reverse Stock Splits

        If the Company shall issue any shares of Common Stock as a stock
dividend or subdivide the number of outstanding shares of Common Stock into a
greater number of shares, then, in either such case, then the Exercise Price in
effect before such dividend or subdivision shall be proportionately reduced and
the number of shares of Warrant Shares at that time purchasable pursuant to this
Warrant shall be proportionately increased; and, conversely, if the Company
shall reduce the number of outstanding shares of Common Stock by combining such
shares into a smaller number of shares, then the Exercise Price in effect before
such combination shall be proportionately increased and the number of shares of
Warrant Shares at that time purchasable pursuant to this Warrant shall be
proportionately decreased. Upon each adjustment in the Exercise Price pursuant
to this Section 5, the number of shares of Warrant Shares purchasable hereunder
shall be adjusted, to the nearest whole share, to the product obtained by
multiplying such number of shares purchasable immediately prior to such
adjustment in the Exercise Price by a fraction, the numerator of which shall be
the Exercise Price immediately prior to such

                                      -5-
<PAGE>

adjustment and the denominator of which shall be the Exercise Price immediately
thereafter. The Holder shall be entitled to the same notice and information
regarding such dividend or subdivision as is furnished to holders of Common
Stock, which notice shall be sent to the Holder no later than the date such
notice is sent to all holders of Common Stock. The foregoing is intended to
protect Holder against dilution due to stock dividends and stock splits.

        5.2 Other Dividends and Distributions

        In case the Company shall take a record of the holders of its Common
Stock (or other stock or securities at the time receivable upon the exercise of
this Warrant) for the purpose of entitling them to receive any dividend or other
distribution other than as described in Section 5.1, or any right to subscribe
for or purchase any shares of stock of any class or any other securities, or to
receive any other right, then the Company will mail or cause to be mailed to the
Holder a notice specifying the date on which a record is to be taken for the
purpose of such dividend, distribution or right (the "Record Date"), and stating
the amount and character of such dividend, distribution or right. Such notice
shall be mailed at least 15 days prior to the Record Date therein specified.

                           SECTION 6 CHANGE OF CONTROL

        Upon a Change of Control (as defined below) of the Company, the
restrictions on exercise as described in Section 1.4 will terminate.

        For the purposes of this Warrant, "Change of Control" of the Company
        means: (i) an acquisition of Common Stock and any other securities
        issued by the Company having the ordinary power to vote in the election
        of directors of the Company (other than securities having such power
        only upon the happening of a contingency) (the "Voting Stock") by any
        person, individual, corporation, partnership, trust or other
        non-governmental entity or any governmental agency, court, authority or
        other body (whether foreign, federal, state, local or otherwise)
        (collectively, "Person") or a Group (as that term has the meaning
        comprehended by Section 13(d)(3) of the Securities Exchange Act of 1934,
        as amended, and the rules and regulations promulgated thereunder (the
        "Exchange Act")) (other than the Company or its affiliates) in a
        purchase or transaction or series of related purchases or transactions
        if immediately thereafter such Person or Group has Beneficial Ownership
        (as such term has the meaning comprehended by Section 13(d)(3) of the
        Exchange Act) of more than fifty percent (50%) of the combined voting
        power of the Company's then outstanding Voting Stock; (ii) the execution
        of an agreement providing for a tender offer, merger, consolidation or
        reorganization, or series of such related transactions involving the
        Company, unless the stockholders of the Company, immediately after such
        transaction or transactions are Beneficial Owners of at least fifty
        percent (50%) of the Voting Stock; (iii) a change or changes in the
        membership of the Company's board of directors that represents a change
        of a majority or more of such membership during any twelve month period
        (unless such change or changes in membership are caused by actions of
        the then existing board of directors and do not occur within twelve
        months of the commencement, threat or proposal of an Election Contest
        (as such term is defined in Rule 14a-11 of Regulation 14A under the
        Exchange Act), tender offer or other transaction that would constitute a
        Change of Control under (i) or (ii) of this paragraph; or (iv) to the
        extent not covered by (i), (ii) or (iii) of this paragraph, any event
        constituting a Reorganization under Section 4.

                                      -6-
<PAGE>

                          SECTION 7 FRACTIONAL SHARES

        No fractional shares shall be issued upon the exercise of this Warrant.
In lieu of fractional shares, the Company shall pay the Holder a sum in cash
equal to the fair market value of the fractional shares on the date of exercise.

                       SECTION 8 RESTRICTIONS ON TRANSFER

        Neither the Security evidenced by this Warrant nor the securities
issuable upon exercise of this Warrant may be transferred unless (a) there is an
effective registration statement under the Securities Act of 1933, as amended
(the "Securities Act"), and applicable state securities laws covering any such
transaction involving said securities, (b) the Company receives opinion of legal
counsel for the holder of said securities (reasonably acceptable to the Company)
stating that such transaction is exempt from registration; provided, however,
that no such opinion of counsel shall be necessary for a transfer of Warrant
Shares pursuant to Rule 144(k) promulgated under the Securities Act or any
successor rule thereto ("Rule 144(k)"), or (c) the Company otherwise satisfies
itself that such transaction is exempt from registration. The Holder and Company
agree that all Warrant Shares shall have the same registration rights and be
subject to the same terms and conditions with respect to the registration and
sale of such stock as provided for in the Amended and Restated Investors' Rights
Agreement dated September 11, 1998 among the Company and the individuals and
entities listed on the signature pages thereto, as amended by the Amendment to
Amended and Restated Investors' Rights Agreement dated January 13, 1999 among
the Company and the individuals and entities listed on the signature pages
thereto (together, the "Rights Agreement"), and as possessed by the individuals
and entities listed on the signature pages to the Rights Agreement.

                                SECTION 9 LEGEND

        A legend setting forth or referring to the foregoing restrictions shall
be placed on this Warrant, any replacement hereof and any certificate
representing a security issued pursuant to the exercise hereof, and a stop
transfer restriction or order shall be placed on the books of the Company and
with any transfer agent until such securities may be legally sold or otherwise
transferred; provided, however, that such legend shall not be required and a
stop transfer restriction order shall not be placed if (a) in the opinion of
counsel to the Holder (reasonably acceptable to the Company) registration of any
future transfer is not required by the applicable provisions of the Securities
Act, (b) the Company shall have waived the requirements of such legends, or (c)
the transfer of Warrant Shares shall be made in compliance with the requirements
of Rule 144(k) or the Company otherwise satisfies itself that such transaction
is exempt from registration.

                           SECTION 10 HOLDER AS OWNER

        The Company may deem and treat the Holder of this Warrant as the
absolute owner hereof for all purposes regardless of any notice to the contrary.

                                      -7-
<PAGE>

                        SECTION 11 WARRANT HOLDER RIGHTS

        This Warrant shall not entitle the Holder to any voting rights or any
other rights as a shareholder of the Company or to any other rights whatsoever
except the rights stated herein; and except as otherwise provided herein, no
dividend or interest shall be payable or shall accrue in respect of this Warrant
or the Warrant Shares purchasable hereunder unless, until and to the extent that
this Warrant shall be exercised.

                             SECTION 12 CONSTRUCTION

        The validity and interpretation of the terms and provisions of this
Warrant shall be governed by the laws of the State of California without respect
to the conflicts of laws principles thereof. The descriptive headings of the
several sections of this Warrant are inserted for convenience only and shall not
control or affect the meaning or construction of any of the provisions thereof.

                              SECTION 13 EXPIRATION

        Subject to the last sentence of Section1.4, any portion of the Warrant
Shares which shall not have become exercisable (in Accordance with the table set
forth in Section 1.1 (a) above) within three years from the date of the Master
Agreement shall be cancelled. Notwithstanding the above, any portion of the
Warrant Shares that have become exercisable (in accordance with the table set
forth in Section 1(a) above) shall be exercisable for a period of five years
commencing on the date such portion of the Warrant Shares became exercisable.
All restrictions set forth herein on the shares of capital stock issued upon
exercise of any rights hereunder shall survive such exercise and expiration of
the rights granted hereunder.

                         SECTION 14 EXCHANGE OF WARRANT

        This Warrant is exchangeable upon the surrender hereof by the Holder at
the office of the Company for new Warrants of like tenor representing in the
aggregate the rights to subscribe for and purchase the number of shares which
may be subscribed for and purchased hereunder, each of such new Warrants to
represent the right to subscribe for and purchase such number of shares as shall
be designated by the Holder at the time of such surrender.

                       SECTION 15 LOST WARRANT CERTIFICATE

        If this Warrant is lost, stolen, mutilated or destroyed, the Company
shall, upon request in writing from the Holder and subject to compliance by
Holder with the following sentence, issue a new Warrant of like denomination,
tenor and date as this Warrant, subject to the Company's right to require the
Holder to give the Company a bond or other satisfactory security sufficient to
indemnify the Company against any claim that may be made against it (including
any expense or liability) on account of the alleged loss, theft, mutilation or
destruction of this Warrant or the issuance of such new Warrant. The Holder
shall reimburse the Company for any and all expenses and costs incurred by the
Company in connection with issuing a new Warrant under this Section.

                                      -8-
<PAGE>

                        SECTION 16 WAIVERS AND AMENDMENTS

        This Warrant or any provision hereof may be changed, waived, discharged
or terminated only by a statement in writing signed by the party against which
enforcement of the change, waiver, discharge or termination is sought.

                        SECTION 17 SUCCESSORS AND ASSIGNS

        This Warrant shall be binding upon the Company and inure to the benefit
of the Holder and its successors and assigns; provided, however, that the
Warrant shall only be assignable by the Holder to its Affiliates (as defined in
the Master Agreement).

                               SECTION 18 NOTICES

        All notices or other communications required or permitted hereunder
shall be in writing and shall be delivered by personal delivery, reputable
overnight courier service, telecopier or mailed by United States mail,
first-class postage prepaid, or by registered or certified mail with return
receipt requested, addressed as follows:

If to the Holder:
U.S. Telesource, Inc.
555 Seventeenth Street
Denver, Colorado 80202
Fax: 303-992-1724
Attention: Legal Counsel

If to the Company:

Critical Path, Inc.
320 First Street
San Francisco, California 94105
Fax: 415-808-8777
Attention: General Counsel

        Each of the foregoing parties shall be entitled to specify a different
address by giving five days' advance written notice as aforesaid to the other
parties. All such notices and communications shall be deemed to have been
received (i) in the case of personal delivery, on the date of such delivery and
(ii) in the case of mailing, on the third business day following the date of
such mailing.

                          SECTION 19 INVESTMENT INTENT

        By accepting this Warrant, the Holder represents that it is acquiring
this Warrant for investment and not with a view to, or for sale in connection
with, any distribution thereof.

                                      -9-
<PAGE>

        IN WITNESS WHEREOF, the Company has executed this Warrant as of the date
first written above.

CRITICAL PATH, INC.

By:
    -----------------------
    Brett Roberston
    Vice President of Strategic Development and
    General Counsel

ACCEPTED AND AGREED:

U.S. TELESOURCE, INC.

By:
    -------------------------------

Name: Marc B. Weisberg
      ----------------------------

Title: President and CEO
       ---------------------------

Date: 10/29/99
      ----------------------------

                                      -10-
<PAGE>

                               NOTICE OF EXERCISE

                             TO CRITICAL PATH, INC.

        The undersigned hereby irrevocably elects to exercise the Warrant
delivered herewith pursuant to Section 1.2 thereof as to __________ shares of
Common Stock and requests that certificates for such shares be issued in the
name of and delivered to the undersigned at the address stated below, and, if
additional shares remain available for purchase pursuant to the Warrant, the new
Warrant evidencing the right to purchase the balance of such shares shall be
registered in the name of, and delivered to, the undersigned at the address
stated below. The undersigned hereby agrees with and represents to the Company
that said shares of common stock are acquired for investment and not with a view
to, or for sale in connection with, any distribution or public offering thereof
within the meaning of the Securities Act of 1933, as amended, and agrees that
the exercise of the Warrant and the issuance and transfer of the common stock to
be purchased are subject to Sections 7 and 8 of the Warrant.

Payment is enclosed in the amount of $
                                      --------------------

Dated:
      --------------------------

--------------------------------

By:
    ----------------------------

Its:
    ----------------------------

Address:

--------------------------------

--------------------------------

--------------------------------

--------------------------------

                                      -11-<PAGE>
                                                                    EXHIBIT 4.4

        THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE BEEN ACQUIRED FOR
INVESTMENT AND HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED (THE "SECURITIES ACT") OR ANY STATE SECURITIES LAWS. SUCH SECURITIES MAY
NOT BE SOLD OR TRANSFERRED IN THE ABSENCE OF SUCH REGISTRATION OR AN EXEMPTION
THEREFROM UNDER THE SECURITIES ACT AND ANY APPLICABLE STATE SECURITIES LAWS.

                           WARRANT TO PURCHASE SHARES
                             OF THE COMMON STOCK OF
                               CRITICAL PATH, INC.

                                DATE: MAY 9, 2001

        This certifies Michael A. LaHorgue and Elizabeth T. LaHorgue, Trustees
of the LAHORGUE FAMILY TRUST dated September 18, 2001 (c/o Seaview Ventures, 60
W. Seaview Avenue, San Rafael, CA 94901) (the "HOLDER"), for value received,
shall be entitled to purchase from Critical Path, Inc., a California corporation
(the "COMPANY"), having its principal place of business at 532 Folsom Street,
San Francisco, California, a maximum of Thirty-Three Thousand Three Hundred and
Thirty Three (33,333) shares of fully paid and assessable shares of the
Company's Common Stock ("COMMON STOCK") for cash, at a purchase price equal to
U.S. Two Dollars and Zero Cents ($2.00) per share (the "EXERCISE PRICE") at any
time, or from time to time, up to and including 5:00 p.m. (local time) on the
third anniversary from the date of this Warrant (the "EXPIRATION DATE"), upon
the surrender to the Company at its principal place of business (or at such
other location as the Company may advise the Holder in writing) of this Warrant
properly endorsed a Form of Subscription in substantially the form attached
hereto duly filled in and signed and upon payment in cash or by check of the
aggregate Exercise Price for the number of shares for which this Warrant is
being exercised determined in accordance with the provisions hereof. The
Exercise Price and the number of shares of Common Stock purchasable hereunder
are subject to adjustment as provided in Section 3 of this Warrant.

        This Warrant is subject to the following terms and conditions:

        1. EXERCISE; ISSUANCE OF CERTIFICATES; PAYMENT FOR SHARES. This Warrant
is exercisable at the option of the holder of record hereof at any time or from
time, to time, up to the Expiration Date for all or any part of the shares of
Common Stock (but not for a fraction of a share) which may be purchased
hereunder, provided however, that Holder must exercise this Warrant for not less
than one hundred (100) shares in each instance. The Company agrees that the
shares of Common Stock purchased under this Warrant shall be and are deemed to
be issued to the Holder hereof as the record owner of such shares as of the
close of business on the date on which this Warrant shall have been surrendered,
properly endorsed, the completed, executed Form of Subscription delivered and
payment made for such shares. Certificates for the shares of Common Stock so
purchased, together with any other securities or property to which the Holder is
entitled upon such exercise, shall be delivered to the Holder by the Company at
the Company's

                                       1.
<PAGE>

expense within a reasonable time after the rights represented by this Warrant
have been so exercised, and in any event, within fifteen (15) days of such
exercise. In case of a purchase of less than all the shares that may be
purchased under this Warrant, the Company shall cancel this Warrant and execute
and deliver a new Warrant or Warrants of like tenor for the balance of the
shares purchasable under the Warrant surrendered upon such purchase to the
Holder hereof within a reasonable time. Each stock certificate so delivered
shall be in such denominations of Common Stock as may be requested by the Holder
hereof, but in no event for less than one hundred (100) shares and shall be
registered in the name designated by such Holder.

        2. SHARES TO BE FULLY PAID; RESERVATION OF SHARES. The Company covenants
and agrees that all shares of Common Stock which may be issued upon the exercise
of the rights represented by this Warrant will, upon issuance, be duly
authorized, validly issued, fully paid and nonassessable and free from all
preemptive rights of any stockholder.. The Company further covenants and agrees
that, during the period within which the rights represented by this Warrant may
be exercised, the Company will at all times have authorized and reserved, for
the purpose of issue or transfer upon exercise of the subscription rights
evidenced by this Warrant, a sufficient number of shares of authorized but
unissued Common Stock, or other securities and property, when and as required to
provide for the exercise of the rights represented by this Warrant. The Company
will take all such action as may be necessary to assure that such shares of
Common Stock may be issued as provided herein without violation of any
applicable law or regulation, or of any requirements of any domestic securities
exchange upon which the Common Stock may be listed; provided, however, that the
Company shall not be required to effect a registration under federal or state
securities laws with respect to such exercise. The Company will not take any
action that would result in any adjustment of the Exercise Price (as set forth
in Section 3 hereof) if the total number of shares of Common Stock issuable
after such action upon exercise of all outstanding warrants, together with all
shares of Common Stock then outstanding and all shares of Common Stock then
issuable upon exercise of all options and upon the conversion of all convertible
securities then outstanding, would exceed the total number of shares of Common
Stock then authorized by the Company's Articles of Incorporation.

        3. ADJUSTMENT OF EXERCISE PRICE AND NUMBER OF SHARES. The Exercise Price
and the number of shares purchasable upon the exercise of this Warrant shall be
subject to adjustment from time to time upon the occurrence of certain events
described in this Section 3. Upon each adjustment of the Exercise Price, the
Holder shall thereafter be entitled to purchase, at the Exercise Price resulting
from such adjustment, the number of shares obtained by multiplying the Exercise
Price in effect immediately prior to such adjustment by the number of shares
purchasable pursuant hereto immediately prior to such adjustment, and dividing
the product thereof by the Exercise Price resulting from such adjustment.

           3.1 SUBDIVISION OR COMBINATION OF STOCK. In case the Company shall at
any time subdivide its outstanding shares of Common Stock into a greater number
of shares, the Exercise Price in effect immediately prior to such subdivision
shall be proportionately reduced, and conversely, in case the outstanding shares
of Common Stock of the Company shall be combined into a smaller number of
shares, the Exercise Price in effect immediately prior to such combination shall
be proportionately increased.

                                       2.
<PAGE>

           3.2 DIVIDENDS IN COMMON STOCK, OTHER STOCK, PROPERTY,
RECLASSIFICATION. If at any time or from time to time the Holders of Common
Stock (or any shares of stock or other securities at the time receivable upon
the exercise of this Warrant) shall have received or become entitled to receive,
without payment therefor,

               (a) Common Stock or any shares of stock or other securities which
are at any time directly or indirectly convertible into or exchangeable for
Common Stock, or any rights or options to subscribe for, purchase or otherwise
acquire any of the foregoing by way of dividend or other distribution,

               (b) any cash paid or payable otherwise than as a cash dividend,
or

               (c) Common Stock or additional stock or other securities or
property (including cash) by way of spinoff, split-up, reclassification,
combination of shares or similar corporate rearrangement, (other than shares of
Common Stock issued as a stock split or adjustments in respect of which shall be
covered by the terms of Section 3.1 above), then and in each such case, the
Holder hereof shall, upon the exercise of this Warrant, be entitled to receive,
in addition to the number of shares of Common Stock receivable thereupon, and
without payment of any additional consideration therefor, the amount of stock
and other securities and property (including cash in the cases referred to in
clause (b) above and this clause (c)) which such Holder would hold on the date
of such exercise had he been the holder of record of such Common Stock as of the
date on which holders of Common Stock received or became entitled to receive
such shares or all other additional stock and other securities and property.

            3.3 REORGANIZATION, RECLASSIFICATION, CONSOLIDATION, MERGER OR SALE.
If any recapitalization, reclassification or reorganization of the capital stock
of the Company, or any consolidation or merger of the Company with another
corporation, or the sale of all or substantially all of its assets or other
transaction shall be effected in such a way that holders of Common Stock shall
be entitled to receive stock, securities, or other assets or property (an
"ORGANIC CHANGE"), then, as a condition of such Organic Change, lawful and
adequate provisions shall be made by the Company whereby the Holder hereof shall
thereafter have the right to purchase and receive (in lieu of the shares of the
Common Stock of the Company immediately theretofore purchasable and receivable
upon the exercise of the rights represented by this Warrant) such shares of
stock, securities or other assets or property as may be issued or payable with
respect to or in exchange for a number of outstanding shares of such Common
Stock equal to the number of shares of such stock immediately theretofore
purchasable and receivable upon the exercise of the rights represented by this
Warrant. In the event of any Organic Change, appropriate provision shall be made
by the Company with respect to the rights and interests of the Holder of this
Warrant to the end that the provisions hereof (including, without limitation,
provisions for adjustments of the Exercise Price and of the number of shares
purchasable and receivable upon the exercise of this Warrant) shall thereafter
be applicable, in relation to any shares of stock, securities or assets
thereafter deliverable upon the exercise hereof. The Company will not effect any
such consolidation, merger or sale unless, prior to the consummation thereof,
the successor corporation (if other than the Company) resulting from such
consolidation or the corporation purchasing such assets shall assume by written
instrument reasonably satisfactory in form and substance to the Holders executed
and mailed or delivered to the registered Holder hereof at the last address of
such Holder appearing on the books of the

                                       3.
<PAGE>

Company, the obligation to deliver to such Holder such shares of stock,
securities or assets as, in accordance with the foregoing provisions, such
Holder may be entitled to purchase.

            3.4 NOTICES OF CHANGE. Immediately upon any adjustment in the number
or class of shares subject to this Warrant and of the Exercise Price, the
Company shall give written notice thereof to the Holder, setting forth in
reasonable detail and certifying the calculation of such adjustment.

        4. ISSUE TAX. The issuance of certificates for shares of Common Stock
upon the exercise of the Warrant shall be made without charge to the Holder of
the Warrant for any issue tax (other than any applicable income taxes) in
respect thereof; provided, however, that the Company shall not be required to
pay any tax which may be payable in respect of any transfer involved in the
issuance and delivery of any certificate in a name other than that of the then
Holder.

        5. CLOSING OF BOOKS. The Company will at no time close its transfer
books against the transfer of any warrant or of any shares of Common Stock
issued or issuable upon the exercise of any warrant in any manner which
interferes with the timely exercise of this Warrant.

        6. NO VOTING OR DIVIDEND RIGHTS; LIMITATION OF LIABILITY. Nothing
contained in this Warrant shall be construed as conferring upon the Holder
hereof the right to vote or to consent or to receive notice as a stockholder of
the Company or any other matters or any rights whatsoever as a stockholder of
the Company. No dividends or interest shall be payable or accrued in respect of
this Warrant or the interest represented hereby or the shares purchasable
hereunder until, and only to the extent that, this Warrant shall have been
exercised. No provisions hereof, in the absence of affirmative action by the
holder to purchase shares of Common Stock, and no mere enumeration herein of the
rights or privileges of the holder hereof, shall give rise to any liability of
such Holder for the Exercise Price or as a stockholder of the Company, whether
such liability is asserted by the Company or by its creditors.

        7. WARRANTS NON-TRANSFERABLE. This Warrant shall not be transferable by
the Holder without the prior written consent of the Company.

        8. RIGHTS AND OBLIGATIONS SURVIVE EXERCISE OF WARRANT. The rights and
obligations of the Company, of the holder of this Warrant and of the holder of
shares of Common Stock issued upon exercise of this Warrant, shall survive the
exercise of this Warrant.

        9. REPRESENTATIONS AND COVENANTS OF THE HOLDER.

           This Warrant has been issued by the Company in reliance upon the
following representations and covenants of the Holder:

               (a) INVESTMENT PURPOSE. The Warrant or the Common Stock issuable
upon exercise of the Warrant (collectively, the "Securities") will be acquired
for investment and not with a view to the sale or distribution of any part
thereof, and the Holder has no present intention of selling or engaging in any
public distribution of the Securities except pursuant to a registration or
exemption.

                                       4.
<PAGE>

               (b) PRIVATE ISSUE. The Holder understands (i) that the Warrant
and the Common Stock issuable upon exercise of this Warrant is not registered
under the Securities Act or qualified under applicable state securities laws on
the ground that the issuance contemplated by this Warrant will be exempt from
the registration and qualifications requirements thereof, and (ii) that the
Company's reliance on such exemption is predicated on the representations set
forth in this Section 9.

               (c) DISPOSITION OF SECURITIES. In no event will the Holder make a
disposition of the Warrant or the Common Stock issuable upon exercise of the
Warrant unless and until (i) it shall have notified the Company of the proposed
disposition, and (ii) if requested by the Company, it shall have furnished the
Company with an opinion of counsel (which counsel may either be inside or
outside counsel to the Holder) satisfactory to the Company and its counsel to
the effect that (A) appropriate action necessary for compliance with the
Securities Act has been taken, or (B) an exemption from the registration
requirements of the Securities Act is available. Notwithstanding the foregoing,
the restrictions imposed upon the transferability of any of its rights to
acquire Common Stock or Common Stock issuable on the exercise of such rights do
not apply to transfers from the beneficial owner of any of the aforementioned
securities to its nominee or from such nominee to its beneficial owner, and
shall terminate as to any particular share of Common Stock when (1) such
security shall have been effectively registered under the Securities Act and
sold by the holder thereof in accordance with such registration or (2) such
security shall have been sold without registration in compliance with Rule 144
under the Securities Act, or (3) a letter shall have been issued to the Holder
at its request by the staff of the Securities and Exchange Commission or a
ruling shall have been issued to the Holder at its request by such Commission
stating that no action shall be recommended by such staff or taken by such
Commission, as the case may be, if such security is transferred without
registration under the Securities Act in accordance with the conditions set
forth in such letter or ruling and such letter or ruling specifies that no
subsequent restrictions on transfer are required. Whenever the restrictions
imposed hereunder shall terminate, as hereinabove provided, the Holder or holder
of a share of Common Stock then outstanding as to which such restrictions have
terminated shall be entitled to receive from the Company, without expense to
such holder, one or more new certificates for the Warrant or for such shares of
Common Stock not bearing any restrictive legend.

               (d) FINANCIAL RISK. The Holder has such knowledge and experience
in financial and business matters as to be capable of evaluating the merits and
risks of its investment, and has the ability to bear the economic risks of its
investment.

               (e) RISK OF NO REGISTRATION. The Holder understands that if a
registration statement covering the Securities under the Securities Act is not
in effect when it desires to sell the Warrant or the Common Stock issuable upon
exercise of the Warrant, it may be required to hold such securities for an
indefinite period. The Holder also understands that any sale of the Warrant or
the Common Stock issuable upon exercise of the Warrant that might be made by it
in reliance upon Rule 144 under the Securities Act may be made only in
accordance with the terms and conditions of that Rule.

        10. "MARKET STAND-OFF" AGREEMENT. The Holder hereby agrees that the
Holder shall not sell or otherwise transfer or dispose of any Common Stock (or
other securities) of the

                                       5.
<PAGE>

Company held the Holder (other than those included in the registration) for a
period specified by the representative of the underwriters of Common Stock (or
other securities) of the Company not to exceed one hundred eighty (180) days
following the effective date of a registration statement of the Company filed
under the Securities Act.

        The Holder further agrees to execute and deliver such other agreements
as may be reasonably requested by the Company or the underwriter which are
consistent with the foregoing or which are necessary to give further effect
thereto. In addition, if requested by the Company or the representative of the
underwriters of Common Stock (or other securities) of the Company, each the
Holder shall provide, within ten (10) days of such request, such information as
may be required by the Company or such representative in connection with the
completion of any public offering of the Company's securities pursuant to a
registration statement filed under the Securities Act. The obligations described
in this Section 11 shall not apply to a registration relating solely to employee
benefit plans on Form S-1 or Form S-8 or similar forms that may be promulgated
in the future, or a registration relating solely to a Commission Rule 145
transaction on Form S-4 or similar forms that may be promulgated in the future.
The Company may impose stop-transfer instructions with respect to the shares of
Common Stock (or other securities) subject to the foregoing restriction until
the end of said one-hundred-eighty (180)-day period.

        11. MODIFICATION AND WAIVER. This Warrant and any provision hereof may
be changed, waived, discharged or terminated only by an instrument in writing
signed by the party against which enforcement of the same is sought.

        12. NOTICES. Any notice, request or other document required or permitted
to be given or delivered to the holder hereof or the Company shall be delivered
or shall be sent by certified mail, postage prepaid, to each such holder at its
address as shown on the books of the Company or to the Company at the address
indicated therefor in the first paragraph of this Warrant or such other address
as either may from time to time provide to the other.

        13. BINDING EFFECT ON SUCCESSORS. This Warrant shall be binding upon any
corporation succeeding the Company by merger, consolidation or acquisition of
all or substantially all of the Company's assets. All of the obligations of the
Company relating to the Common Stock issuable upon the exercise of this Warrant
shall survive the exercise and termination of this Warrant. All of the covenants
and agreements of the Company shall inure to the benefit of the successors and
assigns of the holder hereof.

        14. DESCRIPTIVE HEADINGS AND GOVERNING LAW. The description headings of
the several sections and paragraphs of this Warrant are inserted for convenience
only and do not constitute a part of this Warrant. This Warrant shall be
construed and enforced in accordance with, and the rights of the parties shall
be governed by, the laws of the State of California.

        15. LOST WARRANTS. The Company represents and warrants to the Holder
hereof that upon receipt of evidence reasonably satisfactory to the Company of
the loss, theft, destruction, or mutilation of this Warrant and, in the case of
any such loss, theft or destruction, upon receipt of an indemnity reasonably
satisfactory to the Company, or in the case of any such mutilation upon
surrender and cancellation of such Warrant, the Company, at its expense, will

                                       6.
<PAGE>

make and deliver a new Warrant, of like tenor, in lieu of the lost, stolen,
destroyed or mutilated Warrant.

        16. FRACTIONAL SHARES. No fractional shares shall be issued upon
exercise of this Warrant. The Company shall, in lieu of issuing any fractional
share, pay the holder entitled to such fraction a sum in cash equal to such
fraction multiplied by the then effective Exercise Price.

                      [THIS SPACE INTENTIONALLY LEFT BLANK]

                                       7.
<PAGE>

        IN WITNESS WHEREOF, the Company has caused this Warrant to be duly
executed by its officers, thereunto duly authorized this __ day of ____________,
2001.

                                        CRITICAL PATH, INC.
                                        a California corporation

                                        By:
                                           ------------------------------------

                                        Print Name:

                                        Title: Senior Vice President, General
                                               Counsel and Secretary

                                       8.
<PAGE>

                                    EXHIBIT A

                                SUBSCRIPTION FORM

                                                       Date:
                                                            --------------------

Critical Path, Inc.
532 Folsom Street
San Francisco, CA 94105

Attn:  Chief Financial Officer

Ladies and Gentlemen:

        The undersigned hereby elects to exercise the warrant issued to it by
Critical Path, Inc. (the "COMPANY") and dated ___________________ (the
"WARRANT"), and to purchase thereunder __________________________________ shares
of the Common Stock of the Company (the "SHARES") at a purchase price of Two
Dollars ($2.00) per Share or an aggregate purchase price of
__________________________________ Dollars ($__________) (the "EXERCISE PRICE").

        Pursuant to the terms of the Warrant the undersigned has delivered the
Exercise Price herewith in full in cash or by certified check or wire transfer.

                                          Very truly yours,

                                          -------------------------------------
                                          Company Name

                                          By:
                                             ----------------------------------

                                          Print Name
                                                    ---------------------------

                                          Title:
                                                -------------------------------

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