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                                                                   EXHIBIT 10.16

THE WARRANT REPRESENTED BY THIS CERTIFICATE HAS BEEN ACQUIRED FOR INVESTMENT AND
NEITHER SUCH WARRANT NOR THE SECURITIES ISSUABLE UPON EXERCISE THEREOF HAVE BEEN
REGISTERED UNDER THE SECURITIES ACT OF 1933 AS AMENDED. NONE OF SUCH SECURITIES
MAY BE SOLD, OFFERED FOR SALE, PLEDGED OR HYPOTHECATED IN THE ABSENCE OF AN
EFFECTIVE REGISTRATION STATEMENT AS TO THE SECURITIES UNDER SAID ACT OR LAWS OR
AN OPINION OF COUNSEL OR OTHER EVIDENCE SATISFACTORY TO THE CORPORATION THAT
SUCH REGISTRATION IS NOT REQUIRED.

                                 S1 CORPORATION

                           STOCK SUBSCRIPTION WARRANT

                                                          December 23, 1999

     1.  GENERAL.

THIS CERTIFIES that, for value received, AMERICA ONLINE, INC. ( "AOL"), or
permitted assigns, is entitled to subscribe for and purchase from S1
CORPORATION, a Delaware corporation (the "Corporation"), at any time or from
time to time during the period (the "Exercise Period") commencing with the date
hereof and ending on the fifth anniversary of the date hereof, on the terms and
subject to the provisions hereinafter set forth, up to 84,994 of shares (subject
to adjustment as provided herein) of fully paid and non-assessable shares of
Common Stock, $0.01 par value (the "Common Stock"), of the Corporation, at a
price per share (the "Warrant Price") of $80.00. The shares of capital stock of
the Corporation issuable upon exercise or exchange of this Warrant are sometimes
hereinafter referred to as the "Warrant Shares".

    2.   EXERCISE OF WARRANT.

         The rights represented by this Warrant may be exercised by the holder
hereof in whole or in part, at any time or from time to time during the Exercise
Period, by the surrender of this Warrant (properly endorsed) at the office of
the Corporation at 3390 Peachtree Road NE, Suite 1700, Atlanta, Georgia 30326,
or at such other agency or office of the Corporation in the United States of
America as it may designate by notice in writing to the holder hereof at the
address of such holder appearing on the books of the Corporation, and by payment
(either in cash, by check, or by wire transfer of immediately available funds)
to the Corporation of the

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Warrant Price for each Warrant Share being purchased together with any
applicable taxes. In the event of the exercise of the rights represented by this
Warrant, a certificate or certificates for the Warrant Shares so purchased,
registered in the name of the holder, and if this Warrant shall not have been
exercised for all of the Warrant Shares, a new Warrant, registered in the name
of the holder hereof, of like tenor to this Warrant, shall be delivered to the
holder hereof within a reasonable time, not exceeding ten days, after the rights
represented by this Warrant shall have been so exercised. The person in whose
name any certificate for Warrant Shares is issued upon exercise of this Warrant
shall for all purposes be deemed to have become the holder of record of such
shares on the date on which the Warrant was surrendered and payment of the
Warrant Price and any applicable taxes was made, irrespective of the date of
delivery of such certificate, except that, if the date of such surrender and
payment is a date when the stock transfer books of the Corporation are closed,
such person shall be deemed to have become the holder of such shares at the
close of business on the next succeeding date on which the stock transfer books
are open.

     3.    EXCHANGE OF WARRANT.

           (a) In addition to, and independent of, the rights of the holder of
this Warrant set forth in Section 2 hereof, the holder hereof may at any time or
from time to time elect to receive, without the payment by the holder of any
additional consideration, that number of Warrant Shares determined as
hereinafter provided in this Section 3 by the surrender of this Warrant or any
portion hereof to the Corporation, accompanied by an executed Notice of Exchange
in substantially the form thereof attached hereto (the "Net Issue Election").
Thereupon, the Corporation shall issue to the holder hereof such number of fully
paid and nonassessable Warrant Shares as is computed using the following
formula:

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

where X =  the number of Warrant Shares to be issued to the holder pursuant
           to this Section 3.

      Y =  the number of Warrant Shares covered by this Warrant in respect
           of which the Net Issue Election is made pursuant to this Section
           3.

      A =  the Fair Market Value (as hereinafter defined) of one Warrant
           Share determined at the time the Net Issue Election is made
           pursuant to this Section 3 (the "Determination Date").

      B =  the Warrant Price in effect under this Warrant at the time the
           Net Issue Election is made pursuant to this Section 3.

For purposes of the above calculation, "Fair Market Value" of one Warrant Share
as of the Determination Date shall mean:

              (i)    (A) if the Common Stock is not then traded on a national
securities exchange, the average of the closing prices quoted on the National
Association of Securities Dealers, Inc. Automated Quotation National Market
System, if applicable, or the average of the last bid and asked prices of the
Common Stock quoted in the over-the-counter-market or (B) if the Common Stock is
then traded on a national securities exchange, the average of the high and low
prices of the Common Stock listed on the principal national securities exchange
on which

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the Common Stock is so traded, in each case for the twenty (20) trading days
immediately preceding the Determination Date or, if such date is not a business
day on which shares are traded, the next immediately preceding trading day; and

              (ii) in the event of a Warrant Exchange in connection with a
Corporate Transaction, the value per share of Common Stock received or
receivable by each holder thereof (assuming for purposes of this determination,
in the case of a sale of assets, the Corporation is liquidated immediately
following such sale and the consideration paid to the Corporation is immediately
distributed to its stockholders); and

              (iii) in all other circumstances, the fair market value per share
of Common Stock as determined by the Corporation's Board of Directors in good
faith after taking into consideration all factors it deems appropriate,
including, without limitation, recent sale and offer prices of the capital stock
of the Corporation in private transactions negotiated at arm's length.

The closing of any Warrant Exchange shall take place at the offices of the
Corporation on the date specified in the Notice of Exchange (the "Exchange
Date"), which shall be not less than five and not more than 30 days after the
delivery of such Notice. At such closing, the Corporation shall issue and
deliver to the holder or its designee a certificate or certificates for the
Warrant Shares to be issued upon such Warrant Exchange, registered in the name
of the holder or such designee, and if such Warrant Exchange shall not have been
for all Warrant Shares, a new Warrant, registered in the name of the holder, of
like tenor to this Warrant for the number of shares still subject to this
Warrant following such Warrant Exchange.

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4.      ADJUSTMENT OF WARRANT PRICE.

            (a)    The Warrant Price shall be subject to adjustment from time to
time as follows:

                   (i)    If, at any time during the Exercise Period, the number
        of shares of Common Stock outstanding is increased by a stock dividend
        payable in shares of Common Stock or by a subdivision or split-up of
        shares of Common Stock, then, following the record date fixed for the
        determination of holders of Common Stock entitled to receive such stock
        dividend, subdivision or split-up, the Warrant Price shall be
        appropriately decreased and the number of shares of Common Stock
        issuable upon exercise of this Warrant shall be appropriately increased,
        in each case in proportion to such increase in outstanding shares.

                   (ii)   If, at any time during the Exercise Period, the number
        of shares of Common Stock outstanding is decreased by a combination of
        the outstanding shares of Common Stock, then, following the record date
        for such combination, the Warrant Price shall be appropriately increased
        and the number of shares of Common Stock issuable upon exercise of this
        Warrant shall be appropriately decreased, in each case, in proportion to
        such decrease in outstanding shares.

                   (iii)  All calculations under this Section 4(a) shall be made
        to the nearest one tenth (1/10) of a cent or to the nearest one tenth
        (1/10) of a share, as the case may be.

            (b)    Whenever the Warrant Price shall be adjusted as provided in
this Section 4 the Corporation shall forthwith file, at the office of the
Corporation or any transfer agent designated by the Corporation for the Common
Stock, a statement, signed by its chief financial officer, showing in detail the
facts requiring such adjustment and the adjusted Warrant Price. The Corporation
shall also cause a copy of such statement to be sent by first-class certified
mail, return receipt requested, postage prepaid, to each holder of a Warrant at
his or its address appearing on the Corporation's records. Where appropriate,
such copy may be given in advance and may be included as part of a notice
required to be mailed under the provisions set forth immediately below.

            (c)    In the event the Corporation shall propose to take any action
of the types described in Sections 4(a) or Section 9, the Corporation shall give
notice to each holder of a Warrant in the manner set forth herein, which notice
shall specify the record date, if any, with respect to any such action and the
date on which such action is to take place. Such notice shall also set forth
such facts with respect thereto as shall be reasonably necessary to indicate the
effect of such action (to the extent such effect may be known at the date of
such notice) on the Warrant Price then in effect and the number, kind or class
of shares or other securities or property which shall be delivered or
purchasable upon the occurrence of such action or deliverable upon exercise of
this Warrant. In the case of any action which would require the fixing of a
record date, such notice shall be given at least 20 days prior to the date so
fixed, and in case of all other action, such notice shall be given at least 30
days prior to the taking of such

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proposed action. Failure to give such notice, or any defect therein, shall not
affect the legality or validity of any such action.

     5.  COVENANTS AS TO COMMON STOCK.

         The Corporation 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 validly issued, fully paid and non-assessable and free
from all liens and charges with respect to the issuance thereof. The Corporation
further covenants and agrees that the Corporation will at all times have
authorized and reserved, free from preemptive rights, a sufficient number of
shares of its Common Stock to provide for the exercise of the rights represented
by this Warrant and will in good faith and as expeditiously as possible endeavor
to take all actions necessary to issue such shares in accordance herewith. If
and so long as the Common Stock issuable upon the exercise of this Warrant is
listed on any national securities exchange, the Corporation will, if permitted
by the rules of such exchange, list and keep listed on such exchange, upon
official notice of issuance, all shares of such capital stock.

     6.  NO SHAREHOLDER RIGHTS.

         This Warrant shall not entitle the holder hereof to any voting rights
or other rights as a shareholder of the Corporation.

     7.  RESTRICTIONS ON TRANSFER.

         The holder of this Warrant acknowledges that neither this Warrant nor
the Warrant Shares have been registered under the Securities Act of 1933, as
amended (the "Securities Act") and the holder of this Warrant agrees that no
sale, transfer, assignment, hypothecation or other disposition of this Warrant
or the Warrant Shares shall be made in the absence of (a) current registration
statement under the Securities Act as to this Warrant or the Warrant Shares and
the registration or qualification of this Warrant or the Warrant Shares under
any applicable state securities laws is then in effect or (ii) an opinion of
counsel reasonably satisfactory to the Corporation to the effect that such
registration or qualification is not required. Each certificate or other
instrument for Warrant Shares issued upon exercise of this Warrant shall, if
required under the Securities Act or the rules promulgated thereunder, be
imprinted with a legend substantially to the foregoing effect. No holder of this
Warrant shall sell, transfer, or assign this Warrant, except (i) to such
holder's Affiliate or (ii) with the written consent of the Company. As used
herein, an "Affiliate" of such holder means any person or entity that controls
such holder, is controlled by such holder or is under common control with such
holder.

     8.  TRANSFER OF WARRANT.    Subject to the restrictions set forth in
Section 7, this Warrant and all rights hereunder are transferable, in whole, or
in part, at the agency or office of the Corporation referred to in Section 2, by
the holder hereof in person or by duly authorized attorney, upon surrender of
this Warrant properly endorsed. Each taker and holder of this Warrant, by taking
or holding the same, consents and agrees that this Warrant, when endorsed, in
blank, shall be deemed negotiable, and, when so endorsed the holder hereof may
be treated by the Corporation and all other persons dealing with this Warrant as
the absolute owner hereof for any purposes and as the person entitled to
exercise the rights represented by this Warrant, or to the transfer hereof on
the books of the Corporation, any notice to the contrary notwithstanding;

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but until each transfer on such books, the Corporation may treat the registered
holder hereof as the owner hereof for all purposes.

     9.  REORGANIZATIONS, ETC. In case, at any time during the Exercise Period,
of any capital reorganization, of any reclassification of the stock of the
Corporation (other than a change in par value or from par value to no par value
or from no par value to par value or as a result of a stock dividend or
subdivision, split-up or combination of shares), or the consolidation or merger
of the Corporation with or into another corporation (other than a consolidation
or merger in which the Corporation is the continuing operation and which does
not result in any change in the Common Stock) or of the sale of all or
substantially all the properties and assets of the Corporation as an entirety to
any other corporation, this Warrant shall, after such reorganization,
reclassification, consolidation, merger or sale, be exercisable for the kind and
number of shares of stock or other securities or property of the Corporation or
of the corporation resulting from such consolidation or surviving such merger or
to which such properties and assets shall have been sold to which such holder
would have been entitled if he had held the Common Stock issuable upon the
exercise hereof immediately prior to such reorganization, reclassification,
consolidation, merger or sale. In any such reorganization or other action or
transaction described above, appropriate provision shall be made with respect to
the rights and interests of the holder of this Warrant to the end that the
provisions hereof shall thereafter be applicable, as nearly as may be, in
relation to any shares of stock, securities or assets thereafter deliverable
upon the exercise hereof. The Corporation will not effect any such
consolidation, merger or sale unless, prior to the consummation thereof, the
successor corporation or entity (if other than the Corporation) resulting from
such transaction or the corporation or entity purchasing such assets shall
assume by written instrument, executed and mailed or delivered to the registered
holder hereof at the last address of such holder appearing on the books of the
Corporation, 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.

     10. LOST, STOLEN, MUTILATED OR DESTROYED WARRANT. If this Warrant is lost,
stolen, mutilated or destroyed, the Corporation may, on such terms as to
indemnity or otherwise as it may in its discretion impose (which shall, in the
case of a mutilated Warrant, include the surrender thereof), issue a new Warrant
of like denomination and tenor as the Warrant so lost, stolen, mutilated or
destroyed. Any such new Warrant shall constitute an original contractual
obligation of the Corporation, whether or not the allegedly lost, stolen,
mutilated or destroyed Warrant shall be at any time enforceable by anyone.

     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. All notices, advices and communications to be given or
otherwise made to any party to this Agreement shall be deemed to be sufficient
if contained in a written instrument delivered in person or duly sent by first
class registered or certified mail, return receipt requested, postage prepaid,
or by overnight courier, addressed to such party at the address set forth below
or at such other address as may hereafter be designated in writing by the
addressee to the addresser listing all parties:

         (a)     If to the Corporation, to:

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                  S1 Corporation
                  3390 Peachtree Road NE
                   Suite 1700
                   Atlanta, Georgia 30326:
                  Attention:  Secretary

         (b)     If to AOL as follows:

                 America Online, Inc.
                 22000 AOL Way
                 Dulles, Virginia  20166
                 Attention: General Counsel

Or to such other address as the party to whom notice is to be given may have
furnished to the other parties hereto in writing in accordance herewith. Any
such notice or communication shall be deemed to have been delivered and received
(i) in the case of personal delivery, on the date of such deliver, (ii) in the
case of nationally-recognized overnight courier, on the next business day after
the date when sent and (ii) in the case of mailing, on the third business day
following that on which the piece of mail containing such communication is
posted. As used in this Section 12, "business day" shall mean any day other than
a day on which banking institutions in the Commonwealth of Virginia are legally
required to be closed for business.

     13. BINDING EFFECT ON SUCCESSORS; SURVIVAL. This Warrant shall be binding
upon any corporation succeeding the Corporation by merger, consolidation or
acquisition of all or substantially all of the Corporation's assets. All of the
obligations of the Corporation 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 Corporation shall inure to
the benefit of the successors and assigns of AOL.

     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 Commonwealth of Virginia.

     15. FRACTIONAL SHARES.  No fractional shares shall be issued upon exercise
of this Warrant. The Corporation 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 Fair Market Value of one Warrant Share.

                                     * * *

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               IN WITNESS WHEREOF, the undersigned have caused this Warrant and
Warrant Agreement to be executed by their duly authorized officers on the date
first above written.

                                            S1 CORPORATION

                                            By: /s/ ROBERT F. STOCKWELL
                                                -------------------------------
                                                Name: Robert F. Stockwell
                                                Title: Chief Financial Officer

ATTEST: /s/ NANCY K. KENLEY
        -----------------------
          SECRETARY

                                            AMERICA ONLINE, INC.

                                            By: /s/ DAVID M. COLBURN
                                                -------------------------------
                                                Name: David M. Colburn
                                                Title: President, Business
                                                         Affairs

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                              FORM OF SUBSCRIPTION

                     [To be signed upon exercise of Warrant]

               The undersigned, the holder of the Warrant, hereby irrevocably
elects to exercise the purchase rights represented by such Warrant for, and to
purchase thereunder, _________ shares of _________ of S1 Corporation and
herewith makes payment of $_________ therefor, and requests that the
certificates for such shares be issued in the name of and delivered to,
_________________________________, whose address is

_______________________________________________.

Dated:_____________
                                            ---------------------------------
                                            (Signature)

                                            ---------------------------------
                                            (Address)

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                               NOTICE OF EXCHANGE

                        (To be executed by the Holder in
                         order to exchange the Warrant.)

               The undersigned hereby irrevocably elects to exchange this
Warrant into __________ shares (the foregoing number constituting the number of
Warrant Shares to be issued pursuant to Section 3 of this Warrant) of ________
of S1 Corporation, minus any shares to be deducted from the foregoing number in
accordance with the terms of this Warrant, according to the conditions thereof.
The undersigned desires to consummate such exchange on ________________.

Dated:

                                            -----------------------------
                                            Name of Holder:

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

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                               FORM OF ASSIGNMENT

    [To be signed only upon transfer of Warrant subject to Section 7 thereof]

               For value received, the undersigned hereby sells, assigns and
transfers unto the right represented by the Warrant to purchase _______ shares
of _________ S1 Corporation, to which the Warrant relates, and appoints Attorney
to transfer such right on the books of S1 Corporation, with full power of
substitution in the premises.

Dated:_____________

                                            ----------------------------
                                            (Signature)

Signed in the presence of:

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                                       11<PAGE>   1

                   [SECURITY FIRST TECHNOLOGIES LETTERHEAD]

September 23, 1999

Gregg Freishtat
C/o VerticalOne Corporation
Two Concourse Parkway
Atlanta, GA 30328

This letter is written in contemplation of the proposed acquisition of your
current employer, VerticalOne Corporation ("VerticalOne"), by Security First
Technologies Corporation ("S1"). The proposed employment contemplated by this
letter is conditioned upon, and will only take effect in the event that, S1
completes its acquisition of VerticalOne and you remain, at the time the
acquisition is completed, an employee in good standing with VerticalOne.  If
either of those conditions does not exist, this letter will have no force or
effect.

It is my pleasure to advise you that if the conditions described above are met,
on the closing date of the merger, S1 is prepared to continue your employment
with VerticalOne on the same basis as your employment with VerticalOne on that
date.  Accordingly, upon consummation of the proposed acquisition, you will
continue to have the same title and reporting structure that you have as of the
date of closing, and your existing options to purchase shares of Common Stock of
VerticalOne will be honored as options to purchase shares of S1 Common Stock
(subject only to changes in the number of shares and option price based on the
S1/VerticalOne exchange ratio set forth in the merger agreement). Your base
salary (which is paid by S1 on a bimonthly basis on the 15th and 30th of each
month) will also remain the same.

Our present intention is to consolidate all of S1's employee benefit programs
into a single parent company program on or about January 1, 2000. Accordingly,
if we complete that effort on schedule and if the merger is consummated on or
before January 1, 2000, at that time, you would be entitled to receive the same
benefits afforded to all employees of S1. Until that time, your VerticalOne
benefits would remain in effect. S1's benefits package is quite comprehensive
and includes a variety of coverage (life insurance, medical and dental benefits,
and 401K plan).

In addition to retaining the options you have been granted to date by
VerticalOne, based on the recommendation of Greg Freishtat, the CEO of
VerticalOne, S1 will grant you stock options to purchase 300,000 shares of
Common Stock of S1 at an exercise price equal to the lesser of the fair market
value of a share of S1 Common Stock on the date of execution of the merger
agreement or the date of closing. These options will vest 25% annually, with the
first vesting date being the anniversary of the closing date of the merger.
Consistent with your prior discussions with S1's management, the option
agreement from S1 and the option agreement resulting from the conversion of
VerticalOne options into S1 options will each reflect that 50% of your unvested
options will accelerate and become exercisable as of the effective date of any
"change of control" of S1.

In consideration of this offer of employment, to the extent that you hold shares
of VerticalOne Common Stock on the closing date, you hereby agree with S1 not to
offer, sell, transfer or otherwise dispose of (collectively, "transfer") any
shares of S1 Common Stock received upon conversion of such outstanding shares of
VerticalOne Common Stock pursuant to the merger agreement except as follows: 33%
of the S1 Common Stock received may be sold six months after the closing date of
the merger; 66%, 12 months after the closing date; and 100%, 18 months after the
closing date. These lock-up restrictions will lapse in the event your employment
with VerticalOne is terminated after the merger with S1 "without cause," as
defined below. In addition, by accepting this offer, you
<PAGE>   2
acknowledge that the proposed draft Employment Agreement between you and
VerticalOne ostensibly dated June 18, 1999, was never entered into, and you
agree not to assert any rights thereunder against VerticalOne or S1 in the event
the merger with S1 is consummated.

For purposes of this agreement, the term "without cause" shall be defined to
mean termination of your employment over the objection of VerticalOne's chief
executive officer, as contemplated by Section 6.6(b) of the merger agreement. In
the event of your termination without cause, in addition to the rights set forth
above, you will be entitled to 90 days severance pay and accelerated vesting
with respect to the unvested portion of your remaining options that vests within
one year of the date of termination.

As noted above, this agreement is contingent upon the successful completion of
the merger and your status as an employee of VerticalOne in good standing at
that time. S1 agrees not to revoke this offer for any reason while the merger is
pending.

We are excited at the prospect of having you and your colleagues at VerticalOne
join the S1 family and become key members of our team! Our success as a Company
hinges on the people who make up the team, and we believe you will add
significant value to our efforts. We look forward to working with you. Our
objective is to build and maintain an organization of exceptional people, and to
create an environment that enables each individual to realize his/her maximum
potential. Toward this end, your employment with S1 is voluntarily entered into,
and you are free to resign at any time for any reason. Similarly, the Company
reserves the right to terminate the employment relationship at any time for any
reason. While we hope that our relationship will be long and beneficial, the
Company can give no guarantee or contract, either expressed or implied, of
continued employment.

We are looking forward to working with you and having you begin your employment
with us as soon as the merger is consummated. You may be required to attend an
orientation session sometime during your first week with S1 to familiarize you
with our benefits programs and finalize paperwork as part of the employment
process. In the meantime, if you have any questions regarding benefits or
employment issues, please don't hesitate to contact me directly at (404)
812-6200. I would be happy to address any issues or concerns you may have at
any time.

Sincerely,

/s/ JAMES S. MAHAN
James S. Mahan, III
Chairman and CEO
Security First Technologies Corporation

Please indicate your acceptance of this offer, contingent upon consummation of
the merger agreement, by signing below and returning the original to Human
Resources.

          [SIG]                                          Oct 99
---------------------------------                 ---------------------
Accepted                                                  Date

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