<PAGE>

                                                                    EXHIBIT 4.5

        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 Rajiv Surendra Patel, an individual (c/o Spear Street
Capital, Spear Tower, 35th floor, San Francisco, CA 94105) (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 Sixteen Thousand
and Six Hundred Sixty-Seven (16,667) 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 expense within a reasonable time after the rights represented by
this Warrant have been so



                                       1.
<PAGE>

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.

           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



                                       2.
<PAGE>

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 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.
<PAGE>

           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.

               (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



                                       4.
<PAGE>

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 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



                                       5.
<PAGE>

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



                                       6.
<PAGE>

        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:
                                                -------------------------------