<PAGE>
                                                                   Exhibit 10.21

Christopher Cuddy
Interim Chief Executive Officer and President
Engage, Inc.
100 Brickstone Square
Andover, MA 01810

August 29, 2002

RE:   ENGAGE, INC. -- LETTER AGREEMENT WITH CHRISTOPHER CUDDY
      -------------------------------------------------------

Dear Chris:

         This letter summarizes the terms under which you will work at Engage,
Inc. ("Engage" or the "Company") in connection with the transition of your
duties to a successor and the provision of a severance payment at the conclusion
of your employment with the Company. This letter replaces and supercedes any and
all prior discussions and conversations, whether written or oral, regarding the
matters discussed herein. Provided that you are not terminated for Cause (as
such term is defined in Section 5(a) hereof), you satisfy the duties described
herein, and you sign and return to the Company the Company's severance
documentation, the form of which is provided in APPENDIX A attached to this
letter agreement (the "Release"), the Company will pay you the sums as described
below. By signing and returning this letter, you will be entering into a binding
agreement with the Company and will be agreeing to the terms and conditions set
forth herein.

         1.       ON-GOING RESPONSIBILITIES AS INTERIM PRESIDENT AND CHIEF
EXECUTIVE OFFICER. You agree that you will continue to perform your tasks and
responsibilities as Interim President and Chief Executive Officer of the
Company, except that you shall take no action or execute any agreement, document
or instrument on behalf the Company, without the prior approval of John Barone,
Senior Vice President, Sales and Marketing, Lisa McAlister, Chief Financial
Officer, or Daniel Carroll, General Counsel. You shall cooperate in the
transition of your duties to your successor, Mr. Barone, or such other
individual as designed by the Board of Directors ("Successor"), and the
Successor shall be responsible for all day-to-day operational and strategic
decisions regarding the Company. You shall continue to report to work at your
office consistent with past practice and the continued requirements of your
position at the Company's headquarters each work day during the Retention Period
(as defined below). You shall continue to perform the foregoing tasks and
responsibilities until the earlier of (i) immediately after the closing of any
transaction between the Company and CMGI, Inc ("CMGI") pursuant to which, among
other things, CMGI forgives, either entirely or partially, the outstanding
indebtedness of Engage to CMGI or (ii) October 1, 2002 (the "Retention Period").

         2.       RESIGNATION FORM BOARD OF DIRECTORS. You hereby resign as a
director of the Company, effective immediately. You acknowledge that you have no
authority to represent yourself as a director of the Company to any person or
entity or to act or purport to act in any such capacity.
<PAGE>

         3.       SEVERANCE PAYMENT. In the event that your employment is
terminated by the Company for a reason other than for Cause, either prior to or
after the expiration of the Retention Period, the Company shall, as severance
pay, (i) no later than the Thursday following the date of your return to the
Company of an executed copy of the Release ("Return Date") or no later than the
2nd Thursday following the Return Date if the day of the Return Date is a
Wednesday, make a cash payment to you in an amount of $72,500 (less applicable
taxes and withholding) and (ii) if you elect to continue receiving group health
insurance (medical, dental and vision) pursuant to the federal "COBRA" law, 29
U.S.C. ss. 1161 et seq, cover the cost of your health insurance premiums for the
first six (6) months (following the initial six months of COBRA coverage you
will be responsible for all COBRA related costs going forward) (the "Severance
Payment"). You acknowledge that (i) the Severance Payment is being paid to you
in exchange for the consideration specified in this letter and your execution
and return to the Company of the Release and (ii) is in lieu of any bonus
amounts that you would otherwise have been entitled to receive under any prior
agreement with the Company, including without limitation the offer letter dated
October 21, 2001 between you and the Company. You understand and agree that the
payment of the Severance Payment called for by this Section 3 is contingent on
your execution of the Release (the execution of such Release shall not occur
prior to the Termination Date (as defined in Section 5(b) below)).
Notwithstanding anything to the contrary in this letter agreement, you shall be
ineligible to receive the Severance Payment if you are terminated for Cause. The
Company will continue to pay you your annualized base salary of $145,000 (less
all applicable taxes and withholdings) through to the Termination Date.

         Your current 401(k) contributions may at your discretion continue
through to the Termination Date and the employer match portion of your 401(k)
account will vest in accordance with the Company's 401(k) Summary Plan
Description.

         4.       SOLE REMEDY. The payments to you of the amounts payable under
Section 3 together with payment of up to 80 hours of accrued but unused vacation
time, if any, and reimbursement of valid business expenses, shall constitute the
sole monetary remedy by you in the event of a termination of your employment by
the Company for a reason other than Cause. Your vacation time shall cease
accruing effective as of the Termination Date.

         5.       DEFINITIONS. For purposes of this letter agreement, the
following terms shall have the following meanings:

                  (a)      "Cause" shall mean a good faith finding by the
                  Company of: (i) your commission of any act involving fraud,
                  embezzlement, theft, misrepresentation, dishonesty or moral
                  turpitude; (ii) your indictment for the commission of a
                  material crime on the basis of alleged facts of such a serious
                  nature that the Company has reasonable cause to believe that
                  you cannot effectively discharge your duties and
                  responsibilities, or your indictment for the commission of a
                  material business related crime; (iii) any gross misconduct by
                  you; (iv) failure by you to materially perform the duties of
                  your position; (v) any material breach by you of this
                  agreement, the Nondisclosure and Developments Agreement
                  entered into by and between you and the Company on or around
                  October 18, 2001 and the Non-Competition Agreement entered
                  into by and between you and

<PAGE>

                  the Company on or around October 18, 2001;or (vi) disclosure
                  by you to any third party of any component of the terms of
                  this confidential letter agreement, except for disclosure made
                  by you to your legal, accounting, tax or financial advisors,
                  to members of your immediate family, or to any federal, state
                  or local governmental agency.

                  (b)      "Termination Date" shall mean your last day on the
                  payroll of the Company and shall be the earlier of (i) if
                  applicable, the date you are terminated for Cause or (ii) the
                  last date of the Retention Period.

         6.       NOT AN EMPLOYMENT CONTRACT. This letter agreement does not
constitute a guarantee of employment or contract of employment and your
employment may be terminated by you or the Company at any time and for any
reason.

         7.       CONFIDENTIALITY. You acknowledge that the existence and terms
of this letter agreement are confidential. You agree not to disclose your
impending departure from the Company prior to your Termination Date or the
existence or terms of this agreement to any other person, other than to your
immediate family members, legal counsel, accountants, tax return preparer,
financial planner, or to any federal, state, or local government agency, each of
which shall be informed of the confidentiality obligations imposed by this
agreement. Notwithstanding the foregoing, you may disclose the existence but not
the terms of this agreement to a prospective employer, provided that such
prospective employer shall be informed of the confidentiality obligations
imposed by this agreement.

         8.       NON-DISCLOSURE AND NON-COMPETITION AND NON-SOLICITATION. You
acknowledge and reaffirm your obligation to keep confidential all non-public
information concerning the Company which you acquired during the course of your
employment with the Company, as stated more fully in the Nondisclosure and
Developments Agreement between you and the Company on or around October 18, 2001
executed at the inception of your employment which remains in full force and
effect. You further acknowledge and reaffirm your obligations under the
Non-Competition Agreement between you and the Company on or around October 18,
2001 executed for the benefit of the Company and at the inception of your
employment which also remains in full force and effect.

         9.       AMENDMENT. This letter agreement may be amended or modified
only by a written instrument executed by both the Company and you. You and the
Company acknowledge and agree to the terms and conditions stated herein in
consideration of the representations, warranties, covenants and agreements set
forth herein and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged.

         10.      GOVERNING LAW. This letter agreement shall be governed and
construed in accordance with the laws of the Commonwealth of Massachusetts. Any
action, suit or other legal matter arising under or relating to any provision of
this letter agreement shall be commenced only in a court of the Commonwealth of
Massachusetts (or, if appropriate, a federal

<PAGE>

court located within Massachusetts), and the Company and you each consent to the
jurisdiction of such a court. The Company and you each hereby irrevocably waive
any right to a trial by jury in any action, suit or other legal proceeding
arising under or relating to any provision of this letter agreement.

         11.      SUCCESSORS AND ASSIGNS. This letter agreement shall be binding
upon and inure to the benefit of both parties and their respective successors
and assigns, including any corporation with which or into which the Company may
be merged or which may succeed to its assets or business, provided, however,
that the obligations to you are personal and shall not be assigned by you.

         12.      WAIVERS. No delay or omission by the Company in exercising any
right under this letter agreement shall operate as a waiver of that or any other
right. A waiver or consent given by the Company on any one occasion shall be
effective only in that instance and shall not be construed as a bar or waiver of
any right on any other occasion.

         13.      CAPTIONS. The captions of the sections of this letter
agreement are for convenience of reference only and in no way define, limit or
affect the scope or substance of any section of this letter agreement.

         14.      SEVERABILITY. In case any provisions of this letter agreement
shall be invalid, illegal or otherwise unenforceable, the validity, legality and
enforceability of the remaining provisions shall in no way be affected or
impaired thereby.


By signing below, you acknowledge that you have carefully read this letter
agreement and the document attached as APPENDIX A and understand and agree to
all of their terms. Please return one signed copy of this letter agreement to
me.

Sincerely,


Lisa McAlister
Chief Financial Officer and Treasurer

ACCEPTED:


/s/ Christopher Cuddy                               August 29, 2002
---------------------                               ---------------------
Christopher Cuddy                                   Date

Enclosures

cc:      Daniel Carroll, Esq. (w/ encl.)
         Peter Rice (w/ encl.)


<PAGE>

                                   APPENDIX A

______________, 2002

Christopher Cuddy
Interim Chief Executive Officer and President
Engage, Inc.
100 Brickstone Square
Andover, MA 01810


Dear Chris:

         In connection with the termination of your employment with Engage, Inc.
("Engage" or the "Company") on ___________, 2002, and pursuant to your letter
agreement of August 29, 2002 (the "August Letter Agreement") with Engage, a copy
of which is attached to this letter as ATTACHMENT A, you are eligible to receive
the Severance Payment defined in such August Letter Agreement if you sign and
return this letter agreement to Jill Buckley, Vice President of Human Resources
of the Company, in the enclosed envelope on the Termination Date (as defined
herein). By signing and returning this letter, you will be entering into a
binding agreement with the Company and will be agreeing to the terms and
conditions set forth in the numbered sections below, including the release of
claims set forth in Section 3. Therefore, you are advised to consult with your
attorney before signing this letter.

         If you choose not to sign and return this letter agreement by the
Termination Date, you shall not receive any severance benefits from the Company.
You will, however, receive payment for up to 80 hours of unused vacation time,
if any, accrued through the date of the August Letter Agreement. Also,
regardless of whether you sign this letter, you may elect to continue receiving
group health insurance pursuant to the federal "COBRA" law, 29 U.S.C. ss. 1161
ET SEQ, providing you fulfill all enrollment and participation requirements.
Except as specified in the August Letter Agreement, all COBRA premium costs
shall be paid by you on a monthly basis for as long as, and to the extent that,
you remain eligible for COBRA continuation. You should consult the COBRA
materials to be provided by the Company for details regarding these benefits.
All other benefits, including life insurance and long term disability, will
cease upon your Termination Date. Further, pursuant to the Company's stock
option plan(s), you will have up to one month after your Termination Date to
exercise any vested options to purchase Engage stock that you may have, as
provided for by the plan(s). All unvested options will be cancelled on the
Termination Date.

         If, after reviewing this letter agreement with your attorney, you find
the terms and conditions are satisfactory to you, you should sign and return
this letter to Jill Buckley in the enclosed envelope by the Termination Date.
<PAGE>

         The following numbered sections set forth the terms and conditions
which will apply if you timely sign and return this letter:

1.       TERMINATION DATE -- Your effective date of termination from the Company
         is __________, 2002 (the "Termination Date").

2.       DESCRIPTION OF SEVERANCE BENEFITS - The severance benefits to be paid
         to you if you timely sign and return this letter are described in the
         August Letter Agreement and defined therein as the Severance Payment.

3.       RELEASE -- In consideration of the payment of the Severance Payment,
         which you acknowledge you would not otherwise be entitled to receive,
         but without waiving or releasing either party's rights and obligations
         expressly set forth herein or in the August Letter Agreement, you
         hereby fully, forever, irrevocably and unconditionally release, remise
         and discharge the Company, corporate affiliates, subsidiaries, parent
         companies, and its and their officers, directors, stockholders, agents
         and employees (each in their individual and corporate capacities)
         (hereinafter, the "Released Parties") from any and all claims, charges,
         complaints, demands, actions, causes of action, suits, rights, debts,
         sums of money, costs, accounts, reckonings, covenants, contracts,
         agreements, promises, doings, omissions, damages, executions,
         obligations, liabilities, and expenses (including attorneys' fees and
         costs), of every kind and nature which you ever had or now have against
         the Released Parties arising out of your employment with and/or
         separation from the Company, including, but not limited to, all
         employment discrimination claims under Title VII of the Civil Rights
         Act of 1964, 42 U.S.C. ss. 2000e ET SEQ., the Age Discrimination in
         Employment Act, 29 U.S.C. ss. 621 ET SEQ., the Americans With
         Disabilities Act of 1990, 42 U.S.C., ss. 12101 ET SEQ., the Family and
         Medical Leave Act, 29 U.S.C. ss. 2601 ET SEQ., and the Massachusetts
         Fair Employment Practices Act., M.G.L. c.151B, ss. 1 ET SEQ., all as
         amended; all claims arising out of the Fair Credit Reporting Act, 15
         U.S.C.ss.1681 ET SEQ., the Employee Retirement Income Security Act of
         1974 ("ERISA"), 29 U.S.C. ss. 1001 ET SEQ., the Worker Adjustment and
         Retraining Notification Act, 29 U.S.C. ss. 2101 ET SEQ., the
         Massachusetts Civil Rights Act, M.G.L. c.12 ss. ss. 11H and 11I, the
         Massachusetts Equal Rights Act, M.G.L. c.93, ss. 102 and M.G.L. c.214,
         ss. 1C, the Massachusetts Labor and Industries Act, M.G.L. c.149, ss. 1
         ET SEQ., the Massachusetts Privacy Act, M.G.L. c. 214, ss. 1B, and the
         Massachusetts Maternity Leave Act , M.G.L. c. 149, ss. 105(d), all as
         amended; all common law claims including, but not limited to, actions
         in tort, defamation and breach of contract; all claims to any
         non-vested ownership interest in the Company, contractual or otherwise,
         including but not limited to claims to stock or stock options; and any
         claim or damage arising out of your employment with or separation from
         the Company (including a claim for retaliation) under any common law
         theory or any federal, state or local statute or ordinance not
         expressly referenced above; provided, however, that nothing in this
         Agreement prevents you from filing, cooperating with, or participating
         in any proceeding before the EEOC or a state Fair Employment Practices
         Agency (except that you acknowledge that you may

<PAGE>

         not be able to recover any monetary benefits in connection with any
         such claim, charge or proceeding).

4.       NON-DISCLOSURE AND NON-COMPETITION AND NON-SOLICITATION -- You
         acknowledge and reaffirm your obligation to keep confidential all
         non-public information concerning the Company which you acquired during
         the course of your employment with the Company, as stated more fully in
         the Nondisclosure and Developments Agreement between you and the
         Company on or around October 18, 2001 executed at the inception of your
         employment which remains in full force and effect. You further
         acknowledge and reaffirm your obligations under the Non-Competition
         Agreement between you and the Company on or around October 18, 2001
         executed for the benefit of the Company and at the inception of your
         employment which also remains in full force and effect.

5.       RETURN OF COMPANY PROPERTY -- You confirm that you have returned to the
         Company all keys, files, records (and copies thereof), equipment
         (including, but not limited to, computer hardware, software and
         printers, wireless handheld devices, cellular phones, pagers, etc.),
         Company identification, Company vehicles and any other Company-owned
         property in your possession or control and have left intact all
         electronic Company documents, including but not limited to those which
         you developed or help develop during your employment. You further
         confirm that you have cancelled all accounts for your benefit, if any,
         in the Company's name, including but not limited to, credit cards,
         telephone charge cards, cellular phone and/or pager accounts and
         computer accounts.

6.       NON-DISPARAGEMENT -- You understand and agree that as a condition for
         payment to you of the consideration herein described, you shall not
         make any false, disparaging or derogatory statements regarding the
         Company or any of its directors, officers, employees, agents or
         representatives or about the Company's business affairs and financial
         condition. The Company agrees that it shall use reasonable efforts to
         preclude those of its officers (in their capacities as an officer of
         the Company) privy to the terms of this agreement from making any
         false, disparaging or derogatory statements about you as an officer and
         employee of the Company to any third party. The Company further agrees
         that pursuant to its current policy, responses to direct inquiries
         regarding your employment with the Company or separation from
         employment shall be limited to confirmation of your name, position(s),
         and dates of employment.

7.       AMENDMENT -- This letter agreement shall be binding upon the parties
         and may not be modified in any manner, except by an instrument in
         writing of concurrent or subsequent date signed by duly authorized
         representatives of the parties hereto. This letter agreement is binding
         upon and shall inure to the benefit of the parties and their respective
         agents, assigns, heirs, executors, successors and administrators. You
         and the Company acknowledge and agree to the terms and conditions
         stated herein in consideration of the representations, warranties,
<PAGE>

         covenants and agreements set forth herein and other good and valuable
         consideration, the receipt and sufficiency of which are hereby
         acknowledged.

8.       WAIVER OF RIGHTS -- No delay or omission by the Company in exercising
         any right under this letter agreement shall operate as a waiver of that
         or any other right. A waiver or consent given by the Company on any one
         occasion shall be effective only in that instance and shall not be
         construed as a bar or waiver of any right on any other occasion.

9.       VALIDITY -- Should any provision of this letter agreement be declared
         or be determined by any court of competent jurisdiction to be illegal
         or invalid, the validity of the remaining parts, terms or provisions
         shall not be affected thereby and said illegal or invalid part, term or
         provision shall be deemed not to be a part of this letter agreement.

10.      CONFIDENTIALITY -- You understand and agree that as a condition for
         payment to you of the Severance Payment herein described, the terms and
         contents of this letter agreement, and the contents of the negotiations
         and discussions resulting in this letter agreement, shall be maintained
         as confidential by you and your agents and representatives and shall
         not be disclosed except to the extent required by federal or state law
         or as otherwise agreed to in writing by the Company.

11.      NATURE OF AGREEMENT -- You and the Company understand and agree that
         this letter agreement is a severance agreement and does not constitute
         an admission of liability or wrongdoing on the part of you, the
         Company, or any other person.

12.      ACKNOWLEDGMENTS -- You acknowledge that you have been paid all wages
         and other compensation due and owing through the Termination Date and
         that you have been paid for any and all unused vacation that has been
         accrued through the Termination Date. You acknowledge and understand
         that you shall not be entitled to any payments or benefits from the
         Company other than those expressly set forth in this letter agreement
         and its attachment.

13.      VOLUNTARY ASSENT -- You affirm that no other promises or agreements of
         any kind have been made to or with you by any person or entity
         whatsoever to cause you to sign and return this letter agreement, and
         that you fully understand the meaning and intent of this letter
         agreement. You state and represent that you have had an opportunity to
         fully discuss and review the terms of this letter agreement with an
         attorney. You further state and represent that you have carefully read
         this letter agreement, including ATTACHMENT A, understand the contents
         herein, freely and voluntarily assent to all of the terms and
         conditions hereof, and sign your name of your own free act.

14.      APPLICABLE LAW -- This letter agreement shall be interpreted and
         construed by the laws of the Commonwealth of Massachusetts, without
         regard to conflict of laws provisions. You hereby irrevocably submit to
         and acknowledge and recognize the jurisdiction of the courts of the
         Commonwealth of Massachusetts, or if
<PAGE>

         appropriate, a federal court located in Massachusetts (which courts,
         for purposes of this letter agreement, are the only courts of competent
         jurisdiction), over any suit, action or other proceeding arising out
         of, under or in connection with this letter agreement or the subject
         matter hereof.

15.      ENTIRE AGREEMENT -- This letter agreement, including ATTACHMENT A, and
         the August Letter Agreement and its appendix contain and constitute the
         entire understanding and agreement between the parties hereto with
         respect to your severance benefits and the settlement of claims against
         the Company and cancels all previous oral and written negotiations,
         agreements, commitments, writings in connection therewith. Nothing in
         this section, however, shall modify, cancel or supersede your
         obligations set forth in Section 4 herein.


         If you have any questions about the matters covered in this letter,
please call your Human Resources department.


                                      Very truly yours,

                                      ENGAGE, INC.



                                      By: ________________________________
                                          Jill Buckley
                                          Vice President, Human Resources


         I hereby agree to the terms and conditions set forth above and in the
Description of Retention and Severance Payment, attached hereto as ATTACHMENT A.

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

Christopher Cuddy                                    Date


To be returned in the enclosed envelope by the Termination Date