<PAGE>

                                                                   Exhibit 10.28

          Confidential Materials omitted and filed separately with the
      Securities and Exchange Commission. Asterisks denote such omissions.

                                    ADDENDUM

     This Addendum, dated as of September 1, 2003 (the "Addendum"), and the
related FreeFlow Order Form between the parties dated as of September 1, 2003
(the "New Order Form"), amends and supersedes certain of the provisions of the
(1) Akamai Microsoft Non-Standard Services Agreement Terms & Conditions (the
"Terms & Conditions") entered into by and between Akamai Technologies, Inc.
("Akamai") and Microsoft Corporation ("Microsoft"), dated as of June 29, 2000;
and (2) the Akamai FreeFlow Services Order Form entered into by the parties as
of June 29, 2000 (the "Original Order Form"), as amended to date by the various
Addenda thereto between the parties dated November 1, 2001, December 1, 2002 and
February 1, 2003 (the "Addenda"). Unless otherwise defined herein, all
capitalized terms used herein shall have the meanings assigned to such terms in
the Terms & Conditions or New Order Form.

NOW, THEREFORE, the parties hereby agree as follows:

1.   The New Order Form, its attached Service Level Agreement, and this Addendum
     shall supersede and replace the Original Order Form, including the Addenda
     thereto, effective as of the Billing Effective Date set forth in the New
     Order Form. The rates set forth herein shall apply to any use of FreeFlow
     Services (HTTP delivery or streaming) by a Microsoft Web property,
     including any streaming delivered under the EdgeSuite Streaming Order Form,
     its Exhibits (including the Service Level Agreement) and related Addendum
     between Akamai and Microsoft (MSN Entertainment) with a Billing Effective
     Date of August 1, 2003, copies of which are attached hereto (the "MSN
     Entertainment Order").

2.   Notwithstanding Section 1 above, Microsoft shall have continued use of the
     Microsoft/[**], provided that the rates for usage of the [**] shall be as
     set forth herein.

3.   Microsoft's [**] Usage Commitment for FreeFlow reflected in the New Order
     Form is for HTTP traffic only, including any [**] traffic. No FreeFlow
     streaming traffic will be counted towards such minimum commitment. The [**]
     Usage Commitment shall be comprised of the following regional [**]
     subcommitments: [**] for EMEA and [**] for Asia/ROW (each region as defined
     below).

4.   FreeFlow usage rates, including both HTTP and streaming traffic, shall be
     as follows:

        Domestic:               $[**]/Mbps
        EMEA:                   $[**]/Mbps
        Asia/ROW:               $[**]/Mbps
        [**] (worldwide):       $[**]/Mbps



<PAGE>




     For purposes of the above rates:

     -    All measurements will be taken regionally using separate 95/5
          measurements.

     -    Regions will be determined based on whether Customer Content is served
          to end users in (i) the United States and Canada ("Domestic"), (ii)
          Europe, Middle East and Africa ("EMEA"), or (iii) Asia or the rest of
          the world ("Asia/ROW").

     -    The above prices in this Section 4 and the [**] Usage Commitment in
          Section 3 shall be effective from the first day of the calendar month
          in which this Addendum has been executed by the parties; the pricing
          and usage amount (if any) set forth in the Original Order Form, as
          amended by the Addenda, shall apply for periods prior to such date.

5.   The $[**] per GB pricing for storage reflected in the New Order Form shall
     apply to all Microsoft properties, including MSN Entertainment (see
     Paragraph 7 herein).

6.   During the last month of the first year of the Term of the New Order Form
     (i.e., during August 2004), the parties will negotiate in good faith any
     appropriate modifications to the pricing set forth herein that would apply
     for the second year of the Term. In the event that the parties agree upon
     modified pricing terms, or alternatively agree that no price modifications
     are necessary, they shall execute appropriate documentation and the usage
     commitments set forth herein shall continue for the second year of the
     Term. If the parties are unable to reach agreement on pricing terms during
     such negotiation period then Microsoft may, by providing written notice to
     Akamai prior to September 1, 2004, convert the second year of the Term of
     the New Order Form so that (a) Microsoft's [**] Usage Commitment and any
     regional subcommitments set forth herein are reduced by [**]%, and (b)
     during the second year of the Term the New Order Form and the services
     thereunder shall continue on a month-to-month basis subject to termination
     by either party without penalty by delivery of a written notice of
     termination to the other party at least 10 days prior to the first day of
     the following month.

7.   The parties agree that the MSN Entertainment Order shall remain in full
     force and effect and shall be unaffected by the New Order Form or this
     Addendum except that: (a) the price per GB for storage payable by MSN
     Entertainment shall be as reflected in the New Order Form (i.e., $[**] per
     GB), (b) the term of the MSN Entertainment Order shall be modified so that
     it is co-terminus with the New Order Form and this Addendum, and (c) the
     price for streaming (FreeFlow usage) shall be as set forth in Section 4
     herein.

8.   Akamai agrees that in the event it develops a generally available lower
     cost storage offering, it will offer Microsoft the ability to take
     advantage of such offering.


<PAGE>




9.   For purposes of clarification, the footnote from the Addenda permitting
     Microsoft and Microsoft Affiliates to order FreeFlow services from time to
     time, which is reprinted on Schedule A attached hereto, shall continue to
     apply.

10.  For purposes of clarification, the parties acknowledge that the 30 day
     termination without penalty provision pertaining to FreeFlow Services set
     forth in Section 8.2 of the Terms and Conditions does not apply to the
     FreeFlow Services under the New Order Form.

11.  Akamai has implemented a corporate security policy that is based on the ISO
     17799 standard and is substantially equivalent to the MSN Security Policy
     (version 7.00 revised 1/31/03) provided to Akamai. Akamai will undergo an
     annual assessment in accordance with ISO 17799 by a 'big four' consulting
     firm or a reputable equivalent thereto (currently PricewaterhouseCoopers),
     in either case as selected by Akamai. The costs of the assessment,
     including without limitation all fees charged by such consultants and all
     costs and expenses incurred by Akamai in connection with such assessment,
     shall be borne by Akamai. An executive summary and a formal attestation by
     the consultant will be made available for Microsoft's review. Microsoft
     acknowledges that all information relating to the assessment, including the
     report with respect thereto, the executive summary and the formal
     attestation, shall constitute confidential information subject to the
     non-disclosure provisions of the Terms & Conditions.

     Akamai shall permit Microsoft to review the initial assessment. In the
     event that Microsoft in its reasonable discretion is not satisfied with the
     initial assessment because it shows substantial noncompliance with the ISO
     17799 standard, then Microsoft may terminate without penalty this Addendum
     and the New Order Form by written notice delivered to Akamai within five
     (5) days of the last signature on this Addendum. In the event of a
     termination by Microsoft pursuant to the foregoing sentence, the parties
     shall automatically revert to the Original Order Form, as amended by the
     Addenda.

     With respect to any future assessment made available for review annually by
     Akamai to Microsoft, in the event that Microsoft in its reasonable
     discretion is not satisfied with such assessment because it shows a
     material decrease in compliance with the ISO 17799 standard compared with
     the prior year, then Microsoft shall so notify Akamai within fourteen (14)
     days after receipt of the annual assessment and the parties will negotiate
     in good faith with the goal of developing an action plan designed to
     address any agreed upon deficiencies. In the event that the parties are
     unable to reach agreement on such an action plan then Microsoft may
     terminate without penalty this Addendum and the New Order Form by written
     notice delivered to Akamai within 30 days after Microsoft's initial notice
     regarding non-compliance.

     Akamai will promptly notify a designated Microsoft security contact of any
     security breach or similar incident that has, or might have, compromised
     the privacy or security of any Microsoft end-user personally identifiable
     information in Akamai's possession. In such event, Akamai will keep
     Microsoft updated as to the steps being taken to alleviate any continued
     threat to the privacy or security of such information as well as any
     actions being taken to prevent foreseeable future incidents.

<PAGE>

     The parties acknowledge that Akamai currently does not transmit any
     end-user personally identifiable information on behalf of Microsoft. To the
     extent that Microsoft desires to have Akamai transmit any such information
     in the future it will so notify Akamai and the parties will discuss and
     agree on any appropriate security precautions that may be needed with
     respect to such information.

12.  The Parties agree that they shall work together in good faith to
     incorporate the applicable Microsoft standard terms and conditions for
     vendor services or other mutually agreeable alternatives thereto, including
     the Microsoft corporate payment policy, into the Terms & Conditions, within
     sixty (60) days of the last signature on this Addendum.


     IN WITNESS WHEREOF, and intending to be legally bound hereby, the parties
hereto have caused this Amendment to be duly executed and delivered by their
respective duly authorized officers as of the date and year first above written.


     Agreed and Accepted                             Agreed and Accepted:

     AKAMAI TECHNOLOGIES, INC.                       MICROSOFT CORPORATION

     By: /s/ Mike Carlson                            By: /s/ Steven A. Balmer
         -----------------------                         -----------------------

     Name: Mike Carlson                              Name: Steven A. Balmer
           ---------------------                           ---------------------

     Title: Regional Manager                         Title: CEO
            --------------------                            --------------------

     Date: September 23, 2003                        Date: September 30, 2003
           ---------------------                           ---------------------



<PAGE>




                                   SCHEDULE A


FreeFlow Services may be used by Microsoft and Microsoft Affiliates (as defined
below) whose origin server is located in the territories set forth on Exhibit C
(as may be amended from time to time) provided Akamai and Microsoft accept the
order by executing a separate Order Form for FreeFlow Service which identifies
the Microsoft Affiliate. Akamai hereby grants any such Microsoft Affiliate the
right to use the FreeFlow Services and associated Software in accordance with
the Agreement (including the licensing restrictions and confidentiality
provisions thereof); provided that, so long as Microsoft and Akamai have
accepted the order with respect to a given Microsoft Affiliate as provided for
above, all other rights and obligations under the Agreement shall remain with
Microsoft and Microsoft shall be fully responsible for any breach of the
Agreement by its Affiliates. With respect to any order relating to a given
Microsoft Affiliate Microsoft and Akamai have accepted under the terms hereof,
Akamai shall bill Microsoft and Microsoft shall pay for the FreeFlow Services
used by Microsoft and any such Microsoft Affiliates. Support will be provided
directly to Microsoft for FreeFlow Services used by Microsoft and Microsoft
Affiliates. The parties have waived the requirement to execute a separate Order
Form in order to accept the order with respect to msnbc.com, a Microsoft
Affiliate, and msnbc.com may use the FreeFlow Services in accordance with the
terms of the Agreement.

For purposes of this Agreement, "Affiliate" shall mean any entity now or
hereafter directly or indirectly controlling or controlled by or in common
control with a party, where "control" is defined as the ownership of at least
50% of the equity or beneficial interest of such entity or the right to vote for
or appoint a majority of the board of directors or other governing body of such
entity, and any other entity with respect to which party (i) sold a majority
interest and retained at least 10% ownership of the equity or beneficial
interest of such entity and (ii) has management or operational responsibility
(including the authority to act on behalf of and legally bind such entity)
(collectively "Management or Operational Responsibility") provided that such
entity shall be deemed to be an Affiliate only so long as such "control" exists
or party has Management or Operational Responsibility for such entity. For the
avoidance of doubt, this Agreement shall not apply to any affiliates of
Microsoft Affiliates.


<PAGE>



                                    Exhibit C


                                      [**]

<PAGE>


[AKAMAI LOGO]                                         FREEFLOW ORDER FORM

<TABLE>
<S>                                                                  <C>

===================================================================================================================================
CUSTOMER INFORMATION
                                              BILLING CONTACT INFORMATION:     [X] Centralized Billing or [ ] Decentralized Billing
      Company:  Microsoft Corporation             Contact Name:             Pauline Stuart
      Address:  One Redmond Way                   Address (if different):   1065 LaAvenida; Mountain View CA 94043
                Redmond, WA 98052                 E-Mail: pauline@microsoft.com   Phone: (650) 693-7032     Fax: (650) 693-7020
===================================================================================================================================

===================================================================================================================================
ORDER INFORMATION                                                                                      AKAMAI REP: Mike Carlson

  EFFECTIVE DATE:  09/01/2003    BILLING           09/01/2003              TERM:   2 years          CUSTOMER PO #: ________
                  (mm/dd/yyyy)   EFFECTIVE DATE:  (mm/dd/yyyy)                     (Starts on Billing Date)

                                                                      [ ]  New Service Order (no change to any existing services).
   CUSTOMER TYPE: [X]  Existing Customer           PURPOSE OF ORDER:  [X]  Change Order (change to existing order for this
                  [ ]  New Customer                                             service). Term for underlying existing order
                                                                                will change to the Term listed above.
 AFFILIATE ORDER: [X]  No (use this unless below applies)                       Describe change type here (e.g. renewal,
                  [ ]  Yes customer is incorporating Ts &                       upgrade, add feature, etc.): Upgrade existing
                           Cs signed by an affiliate.                           FreeFlow Service Order and change pricing .
                           Name of affiliate is ________.             [ ]  Transfer from another service to this one (prior
                                                                                service will be terminated on the Billing
                                                                                Effective Date). If selected then list the
                                                                                prior service being terminated here: _______.
===================================================================================================================================

===================================================================================================================================
SERVICE INFORMATION                                                                              ONE TIME FEES        MONTHLY FEES

[X]  FREEFLOW                                                                                                         SEE ADDENDUM

     Includes: HTTP, streaming, and SSL object delivery; Last Mile Accelerator; Basic Cache
     Control; Standard Reporting Tools; Log Delivery Service; Alerts.

         [**] Usage Commitment = [**] aggregate of HTTP traffic. Regional subcommitments and
         pricing as set forth in the Addendum dated the date hereof ("Addendum").

[X]  STORAGE                                                                                                          SEE ADDENDUM

         Base and overage usage Rate = $[**] per GB.

[X]  PREMIUM CUSTOMER CARE AND SUPPORT                                                                                  INCLUDED

[ ]  ACTIVATION -- Integration of new Web properties to be set on a custom basis per property.

                                                                                  TOTAL FEES:          $0             SEE ADDENDUM

SECURITY DEPOSIT:  [ ] Security Deposit (amount equals one month recurring fees for Term of
                       one year or less; two months recurring fees for Term greater than one
                       year) - N/A

One-time fees are billed in advance. Monthly recurring fees are billed each calendar month in advance starting on the Billing Date.
Overage and similar fees are billed in arrears. Services measured in Mbps and Net Storage are billed at 95th percentile of usage or
uncompressed equivalent as measured by Akamai over five minute intervals ("95/5"). Other usage based on total use or uncompressed
equivalent as measured by Akamai. Akamai reserves the right to limit Customer's use of the Akamai streaming network in excess of
Customer's committed usage in the event that force majeure events, such as war, natural disaster or terrorist attack, result in
extraordinary levels of traffic on Akamai's network.
====================================================================================================================================

Customer hereby orders from Akamai the Services described in this Order Form, and the attached Service Level Agreement,
incorporated herein as Exhibit A. This Order Form is being issued  pursuant and subject to the Akamai -- Microsoft Non-Standard
Services Agreement Terms and Conditions between the parties, and shall become valid when executed by Customer and accepted by an
authorized representative of Akamai. Use of any Akamai services and products are subject to Akamai's acceptable use policy
(available on WWW.AKAMAI.COM).

===================================================================================================================================
ACCEPTED BY CUSTOMER:                                                ACCEPTED BY AKAMAI:

SIGNATURE  /s/ ARNE JOSEFSBERG                                       SIGNATURE  /s/ MICHAEL CARLSON

----------------------------------------------------------------     ---------------------------------------------------------------
NAME  ARNE JOSEFSBERG                                                NAME  MICHAEL CARLSON
DATE (mm/dd/yyyy)  September 29, 2003                                DATE (mm/dd/yyyy)   September 23, 2003

----------------------------------------------------------------     ---------------------------------------------------------------
TITLE                                                                TITLE
CUSTOMER CONTACT FOR NOTICES (IF DIFFERENT FROM ADDRESS AT TOP)      AKAMAI CONTACT FOR NOTICES
COMPANY:                                                             AKAMAI TECHNOLOGIES, INC.
ADDRESS:                                                             8 CAMBRIDGE CENTER, CAMBRIDGE, MA 02142 USA
ATTENTION: GENERAL MANAGER, MSN INFRASTRUCTURE PLATFORM SERVICES
COPY TO: LAW & CORPORATE AFFAIRS                                     ATTN: GENERAL COUNSEL
====================================================================================================================================
</TABLE>

<PAGE>



                                    EXHIBIT A
                            CONTENT DELIVERY SERVICES
                             SERVICE LEVEL AGREEMENT

This Service Level Agreement is a continuation of the FreeFlow Order Form dated
as of September 1, 2003, and the related Akamai - Microsoft Non-Standard
Services Agreement Terms & Conditions (the "Agreement") between Microsoft
Corporation ("Customer") and Akamai Technologies, Inc. ("Akamai"). All
capitalized terms not defined herein shall have the meanings that are ascribed
in the Agreement.

I.   SERVICE LEVELS AND PENALTIES

     Akamai agrees to provide a level of service demonstrating:

     (a)  100% Uptime: The Service will serve content 100% of the time.
     (b)  Measurable Performance Enhancement: The Service will deliver content
          measurably faster than the Customer's web site.
     (c)  Credits: If the Service fails to meet either of the above service
          levels, the Customer will receive (as its sole remedy) a credit equal
          to fees for the day in which the failure occurs.

II.  SLA MONITORING METHODOLOGY

     The following methodology will be employed to measure the Service
     availability and performance enhancement:

     Agents and Polling Frequency

     (a)  From six (6) geographically and network-diverse locations in major
          metropolitan areas, Akamai will simultaneously poll a test file
          residing on the Customer's production servers and on Akamai's network
     (b)  The polling mechanism will perform two (2) simultaneous http GET
          operations:

          A test file will be placed on the customer's origin server (ie,
          origin.customer.com).

          One GET operation will be performed to retrieve the file directly from
          the origin server (ie, http://origin.customer.com/testobject).

          The other GET operation will be performed to retrieve the file through
          the Service, by requesting the object from the appropriate customer
          hostname CNAMEd to Akamai (ie, http://www.customer.com/testobject,
          where www.customer.com is CNAMEd to Akamai and configured to pull
          content from origin.customer.com)

     (c)  The Akamaized test content must use a TTL of 2 hours or greater.
     (d)  The test content will be a file of approximately 10 KB in size.
     (e)  Polling will occur at approximately 12-minute intervals.
     (f)  Based on the http GET operations described in II(b) above, the
          response times received from the two sources, (a) the Customer server,
          and (b) the Akamai

<PAGE>

          network, will be compared for the purpose of measuring performance
          metrics and outages.

III. PERFORMANCE METRIC

     The performance metric will be based on a daily average of performance for
     the Service and the Customer's production web server, computed from data
     captured across all regions and hits. Each time will be weighted to reflect
     peak traffic conditions or "primetime" usage. The primetime period is 10 AM
     to 8 PM EST. All times recorded during this period will be weighted by a
     factor of three. If on a given day the Akamai weighted daily average time
     exceeds the Customer's weighted daily average time, then the Customer will
     receive (as its sole remedy) a credit equivalent to fees for that day of
     service.

IV.  OUTAGES

     An outage is defined as a 12-minute period of consecutive failed attempts
     by a single agent to "get" a file from the network while succeeding to
     "get" the test file from the Customer web site. If an outage is identified
     by this method, the customer will receive (as its sole remedy) a credit
     equivalent to the fees for the day in which the failure occurred.

V.   SLA ACTIVATION

     In order to activate the Content Delivery Service Level Agreement, the
     Customer must enter two valid test files for the same object (as described
     in II(c) and (d) above) into the SLA Activation Tool located in the
     Provisioning Center on I.AM.AKAMAI (Akamai's Customer Portal). Detailed
     instructions are provided with the SLA Activation Tool on I.AM.AKAMAI; in
     addition, assistance is available from the Customer's Account Manager. The
     SLA will go into effect within five (5) business days after the Customer
     enters valid test files into the SLA Activation Tool.