<PAGE>   1
                                                                    EXHIBIT 10.1

                          1992 ISOCOR STOCK OPTION PLAN
                              AS AMENDED MAY, 1998

        1. Purposes of the Plan. The purposes of this Stock Option Plan are to
attract and retain the best available personnel for positions of substantial
responsibility, to provide additional incentive to Employees and Consultants of
the Company and its Subsidiaries and to promote the success of the Company's
business. Options granted under the Plan may be incentive stock options (as
defined under Section 422 of the Code) or non-statutory stock options, as
determined by the Administrator at the time of grant of an option and subject to
the applicable provisions of Section 422 of the Code, as amended, and the
regulations promulgated thereunder.

        2. Definitions. As used herein, the following definitions shall apply:

               (a) "Administrator" means the Board or any of its Committees
appointed pursuant to paragraph (a) of Section 4 of the Plan.

               (b) "Board" means the Board of Directors of the Company.

               (c) "Code" means the Internal Revenue Code of 1986, as amended.

               (d) "Committee" means the Committee appointed by the Board of
Directors in accordance with paragraph (a) of Section 4 of the Plan.

               (e) "Common Stock" means the Common Stock of the Company.

               (f) "Company" means ISOCOR, a California corporation.

               (g) "Consultant" means any person, including an advisor, who is
engaged by the Company or any Parent or Subsidiary to render consulting services
and is compensated for such services, and any director of the Company whether
compensated for such services or not, provided that if and in the event the
Company registers any class of any equity security pursuant to the Exchange Act,
the term Consultant shall thereafter not include directors who are not
compensated for their services or are paid only a director's fee by the Company.

               (h) "Continuous Status as an Employee" means the absence of any
interruption or termination of the employment relationship by the Company or any
Subsidiary. Continuous Status as an Employee shall not be considered interrupted
in the case of: (i) sick leave; (ii) military leave; (iii) any other leave of
absence approved by the Board, provided that such leave is for a period of not
more than ninety (90) days, unless reemployment upon the expiration of such
leave is guaranteed by contract or statute, or unless provided otherwise
pursuant to Company policy adopted from time to time; or (iv) in the case of
transfers between locations of the Company or between the Company, its
Subsidiaries or its successor.

               (i) "Employee" means any person, including officers and
directors, employed by the Company or any Parent or Subsidiary of the Company.
The payment of a director's fee by the Company shall not be sufficient to
constitute "employment" by the Company.

               (j) "Exchange Act" means the Securities Exchange Act of 1934, as
amended.

<PAGE>   2

               (k) "Fair Market Value" means, as of any date, the value of
Common Stock determined as follows:

                    (i) If the Common Stock is listed on any established stock
exchange or a national market system including without limitation the National
Market of the National Association of Securities Dealers, Inc. Automated
Quotation ("Nasdaq") System, its Fair Market Value shall be the closing sales
price for such stock (or the closing bid, if no sales were reported, as quoted
on such system or exchange, or the exchange with the greatest volume of trading
in Common Stock) for the last market trading day prior to the time of
determination, as reported in The Wall Street Journal or such other source as
the Administrator deems reliable;

                    (ii) If the Common Stock is quoted on the Nasdaq System (but
not on the National Market thereof) or regularly quoted by a recognized
securities dealer but selling prices are not reported, its Fair Market Value
shall be the mean between the high bid and low asked prices for the Common Stock
for the last market trading day prior to the time of determination, as reported
in The Wall Street Journal or such other source as the Administrator deems
reliable; or

                    (iii) In the absence of an established market for the Common
Stock, the Fair Market Value thereof shall be determined in good faith by the
Administrator.

               (l) "Incentive Stock Option" means an Option intended to qualify
as an incentive stock option within the meaning of Section 422 of the Code.

               (m) "Named Executive" means (any individual who, on the last day
of the Company's fiscal year, is the chief executive officer of the Company (or
is acting in such capacity) or among the four highest compensated officers of
the Company (other than the chief executive officer). Such officer status shall
be determined pursuant to the executive compensation disclosure rules under the
Exchange Act.

               (n) "Nonstatutory Stock Option" means an Option not intended to
qualify as an Incentive Stock Option.

               (o) "Option" means a stock option granted pursuant to the Plan.

               (p) "Optioned Stock" means the Common Stock subject to an Option.

               (q) "Optionee" means an Employee or Consultant who receives an
Option.

               (r) "Parent" means a "parent corporation", whether now or
hereafter existing, as defined in Section 424(e) of the Code.

               (s) "Plan" means this 1992 Stock Option Plan.

               (t) "Share" means a share of the Common Stock, as adjusted in
accordance with Section 13 of the Plan.

               (u) "Subsidiary" means a "subsidiary corporation", whether now or
hereafter existing, as defined in Section 424(f) of the Code.

        3. Stock Subject to the Plan. Subject to the provisions of Section 13 of
the Plan, the maximum aggregate number of shares that may be optioned and sold
under the Plan is 2,600,000 shares of Common Stock. The shares may be
authorized, but unissued, or reacquired Common Stock.


                                      -2-
<PAGE>   3

               If an Option should expire or become unexercisable for any reason
without having been exercised in full, the unpurchased Shares that were subject
thereto shall, unless the Plan shall have been terminated, become available for
future grant under the Plan.

        4. Administration of the Plan.

               (a) Composition of Administrator.

                    (i) Multiple Administrative Bodies. If permitted by Rule
16b-3, and by the legal requirements relating to the administration of incentive
stock option plans, if any, of applicable securities laws and the Code
(collectively, the "Applicable Laws"), the Plan may (but need not) be
administered by different administrative bodies with respect to directors,
officers who are not directors and Employees who are neither directors nor
officers.

                    (ii) Administration with respect to Directors and Officers.
With respect to grants of Options to Employees or Consultants who are also
officers or directors of the Company, the Plan shall be administered by (A) the
Board, if the Board may administer the Plan in compliance with Rule 16b-3 as it
applies to a plan intended to qualify thereunder as a discretionary plan and
Section 162(m) of the Code as it applies so as to qualify grants of Options to
Named Executives as performance-based compensation, or (B) a Committee
designated by the Board to administer the Plan, which Committee shall be
constituted in such a manner as to permit the Plan to comply with Rule 16b-3 as
it applies to a plan intended to qualify thereunder as a discretionary plan, to
qualify grants of Options to Named Executives as performance-based compensation
under Section 162(m) of the Code and otherwise so as to satisfy the Applicable
Laws.

                    (iii) Administration with respect to Other Persons. With
respect to grants of Options to Employees or Consultants who are neither
directors nor officers of the Company, the Plan shall be administered by (A) the
Board or (B) a Committee designated by the Board, which Committee shall be
constituted in such a manner as to satisfy the Applicable Laws.

                    (iv) General. If a Committee has been appointed pursuant to
subsection (ii) or (iii) of this Section 4(a), such Committee shall continue to
serve in its designated capacity until otherwise directed by the Board. From
time to time the Board may increase the size of any Committee and appoint
additional members thereof, remove members (with or without cause) and appoint
new members in substitution therefor, fill vacancies (however caused) and remove
all members of a Committee and thereafter directly administer the Plan, all to
the extent permitted by the Applicable Laws and, in the case of a Committee
appointed under subsection (ii), to the extent permitted by Rule 16b-3 as it
applies to a plan intended to qualify thereunder as a discretionary plan, and to
the extent required under Section 162(m) of the Code to qualify grants of
Options to Named Executives as performance-based compensation.

               (b) Powers of the Administrator. Subject to the provisions of the
Plan and in the case of a Committee, the specific duties delegated by the Board
to such Committee, the Administrator shall have the authority, in its
discretion:

                    (i) to determine the Fair Market Value of the Common Stock,
in accordance with Section 2(k) of the Plan;

                    (ii) to select the Consultants and Employees to whom Options
may from time to time be granted hereunder;

                    (iii) to determine whether and to what extent Options are
granted hereunder;

                                      -3-
<PAGE>   4

                    (iv) to determine the number of shares of Common Stock to be
covered by each such award granted hereunder;

                    (v) to approve forms of agreement for use under the Plan;

                    (vi) to determine the terms and conditions, not inconsistent
with the terms of the Plan, of any award granted hereunder (including, but not
limited to, the share price and any restriction or limitation);

                    (vii) to determine whether and under what circumstances an
Option may be settled in cash under Section 10(f) instead of Common Stock;

                    (viii) to reduce the exercise price of any Option to the
then current Fair Market Value if the Fair Market Value of the Common Stock
covered by such Option shall have declined since the date the Option was
granted.

               (c) Effect of Administrator's Decision. All decisions,
determinations and interpretations of the Administrator shall be final and
binding on all Optionees and any other holders of any Options.

        5. Eligibility.

               (a) Nonstatutory Stock Options may be granted to Employees and
Consultants. Incentive Stock Options may be granted only to Employees. An
Employee or Consultant who has been granted an Option may, if he or she is
otherwise eligible, be granted an additional Option or Options.

               (b) Each Option shall be designated in the written option
agreement as either an Incentive Stock Option or a Nonstatutory Stock Option.
However, notwithstanding such designations, to the extent that the aggregate
Fair Market Value of the Shares with respect to which Options designated as
Incentive Stock Options are exercisable for the first time by any Optionee
during any calendar year (under all plans of the Company or any Parent or
Subsidiary) exceeds $100,000, such excess Options shall be treated as
Nonstatutory Stock Options.

               (c) For purposes of Section 5(b), Incentive Stock Options shall
be taken into account in the order in which they were granted, and the Fair
Market Value of the Shares shall be determined as of the time the Option with
respect to such Shares is granted.

               (d) The Plan shall not confer upon any Optionee any right with
respect to continuation of employment or consulting relationship with the
Company, nor shall it interfere in any way with an Optionee's right or the
Company's right to terminate his or her employment or consulting relationship at
any time, with or without cause.

        6. Term of Plan. The Plan shall become effective upon the earlier to
occur of its adoption by the Board of Directors or its approval by the
shareholders of the Company as described in Section 19 of the Plan. It shall
continue in effect for a term of ten (10) years unless sooner terminated under
Section 15 of the Plan.

        7. Term of Option. The term of each Option shall be the term stated in
the Option Agreement; provided, however, that the term shall be no more than ten
(10) years from the date of grant thereof or such shorter term as may be
provided in the Option Agreement. However, in the case of an Option granted to
an Optionee who, at the time the Option is granted, owns stock representing more
than ten percent (10%) of the voting power of all classes of stock of the
Company or any Parent or Subsidiary, the term of the Option shall

                                      -4-
<PAGE>   5

be five (5) years from the date of grant thereof or such shorter term as may be
provided in the Option Agreement.

        8. Limitation on Grants to Employees. Subject to adjustment as provided
in Section 13 of this Plan, the maximum number of shares which may be subject to
Options granted to any one employee under this Plan for any fiscal year of the
Company shall be 750,000.

        9. Option Exercise Price and Consideration.

               (a) Exercise Price. The per share exercise price for the Shares
to be issued pursuant to exercise of an Option shall be such price as is
determined by the Board, but shall be subject to the following:

                    (i) In the case of an Incentive Stock Option

                         (A) granted to an Employee who, at the time of the
grant of such Incentive Stock Option, owns stock representing more than ten
percent (10%) of the voting power of all classes of stock of the Company or any
Parent or Subsidiary, the per Share exercise price shall be no less than 110% of
the Fair Market Value per Share on the date of grant.

                         (B) granted to any other Employee, the per Share
exercise price shall be no less than 100% of the Fair Market Value per Share on
the date of grant.

                    (ii) In the case of a Nonstatutory Stock Option

                         (A) granted to a person who, at the time of the grant
of such Option, owns stock representing more than ten percent (10%) of the
voting power of all classes of stock of the Company or any Parent or Subsidiary,
the per Share exercise price shall be no less than 110% of the Fair Market Value
per Share on the date of the grant.

                         (B) granted to a person, who, at the time of grant of
such Option is a Named Executive of the Company, the per share exercise price
shall be no less than 100% of the Fair Market Value on the date of grant.

                         (C) granted to any person other than those persons
described in subsections (ii) (A) and (B) above, the per Share exercise price
shall be no less than 85% of the Fair Market Value per Share on the date of
grant.

               (b) Permissible Consideration. The consideration to be paid for
the Shares to be issued upon exercise of an Option, including the method of
payment, shall be determined by the Administrator (and, in the case of an
Incentive Stock Option, shall be determined at the time of grant) and may
consist entirely of (1) cash, (2) check, (3) promissory note, (4) other Shares
that (x) in the case of Shares acquired upon exercise of an Option, have been
owned by the Optionee for more than six months on the date of surrender, and (y)
have a Fair Market Value on the date of surrender equal to the aggregate
exercise price of the Shares as to which such Option shall be exercised, (5)
authorization from the Company to retain from the total number of Shares as to
which the Option is exercised that number of Shares having a Fair Market Value
on the date of exercise equal to the exercise price for the total number of
Shares as to which the Option is exercised, (6) delivery of a properly executed
exercise notice together with such other documentation as the Administrator and
the broker, if applicable, shall require to effect an exercise of the Option and
delivery to the Company of the sale or loan proceeds required to pay the
exercise price, (7) delivery of a subscription agreement for the Shares that
irrevocably obligates the optionholder to take and pay for the Shares not more
than twelve months after the date of delivery of the subscription agreement, (8)
any combination of the foregoing methods of

                                      -5-
<PAGE>   6

payment, or (9) such other consideration and method of payment for the issuance
of Shares to the extent permitted under Applicable Laws. In making its
determination as to the type of consideration to accept, the Board shall
consider if acceptance of such consideration may be reasonably expected to
benefit the Company.

        10. Exercise of Option.

               (a) Procedure for Exercise; Rights as a Shareholder. Any Option
granted hereunder shall be exercisable at such times and under such conditions
as determined by the Board, including performance criteria with respect to the
Company and/or the Optionee, and as shall be permissible under the terms of the
Plan.

                    An Option may not be exercised for a fraction of a Share.

                    An Option shall be deemed to be exercised when written
notice of such exercise has been given to the Company in accordance with the
terms of the Option by the person entitled to exercise the Option and the
Company has received full payment for the Shares with respect to which the
Option is exercised. Full payment may, as authorized by the Board, consist of
any consideration and method of payment allowable under Section 9(b) of the
Plan. Until the issuance (as evidenced by the appropriate entry on the books of
the Company or of a duly authorized transfer agent of the Company) of the stock
certificate evidencing such Shares, no right to vote or receive dividends or any
other rights as a shareholder shall exist with respect to the Optioned Stock,
notwithstanding the exercise of the Option. The Company shall issue (or cause to
be issued) such stock certificate promptly upon exercise of the Option. No
adjustment will be made for a dividend or other right for which the record date
is prior to the date the stock certificate is issued, except as provided in
Section 13 of the Plan.

                    Exercise of an Option in any manner shall result in a
decrease in the number of Shares which thereafter may be available, both for
purposes of the Plan and for sale under the Option, by the number of Shares as
to which the Option is exercised.

               (b) Termination of Employment. In the event of termination of an
Optionee's consulting relationship or Continuous Status as an Employee with the
Company, such Optionee may, but only within three (3) months (or such other
period of time as is determined by the Board, with such determination in the
case of an Incentive Stock Option being made at the time of grant of the Option
and not exceeding three (3) months) after the date of such termination (but in
no event later than the expiration date of the term of such Option as set forth
in the Option Agreement), exercise his or her Option to the extent that Optionee
was entitled to exercise it at the date of such termination. To the extent that
Optionee was not entitled to ,exercise the Option at the date of such
termination, or if Optionee does not exercise such Option to the extent so
entitled within the time specified herein, the Option shall terminate.

               (c) Disability of Optionee. Notwithstanding the provisions of
Section 10(b) above, in the event of termination of an Optionee's consulting
relationship or Continuous Status as an Employee as a result of his or her total
and permanent disability (within the meaning of Section 22(e)(3) of the Code),
Optionee may, but only within twelve (12) months from the date of such
termination (but in no event later than the expiration date of the term of such
Option as set forth in the Option Agreement), exercise the Option to the extent
otherwise entitled to exercise it at the date of such termination. To the extent
that Optionee was not entitled to exercise the Option at the date of
termination, or if Optionee does not exercise such Option to the extent so
entitled within the time specified herein, the Option shall terminate.

               (d) Death of Optionee. In the event of the death of an Optionee,
the Option may be exercised, at any time within twelve (12) months following the
date of death (but in no event later than the expiration date of the term of
such Option as set forth in the Option Agreement), by the Optionee's estate or

                                      -6-
<PAGE>   7

by a person who acquired the right to exercise the Option by bequest or
inheritance, but only to the extent the Optionee was entitled to exercise the
Option at the date of death. To the extent that the Optionee was not entitled to
exercise the Option at the date of termination, or if Optionee does not exercise
such Option to the extent so entitled within the time specified herein, the
Option shall terminate.

               (e) Rule 16b-3. Options granted to persons subject to Section
16(b) of the Exchange Act must comply with Rule 16b-3 and shall contain such
additional conditions or restrictions as may be required thereunder to qualify
for the maximum exemption from Section 16 of the Exchange Act with respect to
Plan transactions.

               (f) Buyout Provisions. The Administrator may at any time offer to
buy out for a payment in cash or Shares, an Option previously granted, based on
such terms and conditions as the Administrator shall establish and communicate
to the Optionee at the time that such offer is made.

        11. Non-Transferability of Options. The Option may not be sold, pledged,
assigned, hypothecated, transferred, or disposed of in any manner other than by
will or by the laws of descent or distribution and may be exercised, during the
lifetime of the Optionee, only by the Optionee.

        12. Stock Withholding to Satisfy Withholding Tax Obligation. At the
discretion of the Administrator. Optionees may satisfy withholding obligations
as provided in this paragraph. When an Optionee incurs tax liability in
connection with an Option, which tax liability is subject to tax withholding
under applicable tax laws, and the Optionee is obligated to pay the Company an
amount required to be withheld under applicable tax laws, the Optionee may
satisfy the withholding tax obligation by electing to have the Company withhold
from the Shares to be issued upon exercise of the Option that number of Shares
having a Fair Market Value equal to the amount required to be withheld. The Fair
Market Value of the Shares to be withheld shall be determined on the date that
the amount of tax to be withheld is to be determined (the "Tax Date").

        All elections by an Optionee to have Shares withheld for this purpose
shall be made in writing in a form acceptable to the Administrator and shall be
subject to the following restrictions:

               (a) the election must be made on or prior to the applicable Tax
Date;

               (b) once made, the election shall be irrevocable as to the
particular Shares of the Option as to which the election is made;

               (c) all elections shall be subject to the consent or disapproval
of the Administrator;

               (d) if the Optionee is subject to Section 16 of the Exchange Act,
the election must comply with the applicable provisions of Rule 16b-3 and shall
be subject to such additional conditions or restrictions as may be required
thereunder to qualify for the maximum exemption from Section 16 of the Exchange
Act with respect to Plan transactions.

        In the event the election to have Shares withheld is made by an Optionee
and the Tax Date is deferred under Section 83 of the Code because no election is
filed under Section 83(b) of the Code, the Optionee shall receive the full
number of Shares with respect to which the Option or Stock Purchase Right is
exercised but, such Optionee shall be unconditionally obligated to tender back
to the Company the proper number of Shares on the Tax Date.

                                      -7-
<PAGE>   8

        13. Adjustments Upon Changes in Capitalization; Corporate Transactions.

               (a) Changes in Capitalization. Subject to any required action by
the shareholders of the Company, the number of shares of Common Stock covered by
each outstanding Option, and the number of shares of Common Stock that have been
authorized for issuance under the Plan but as to which no Options have yet been
granted or that have been returned to the Plan upon cancellation or expiration
of an Option, the maximum number of shares of Common Stock for which Options may
be granted to any employee under Section 8 of the Plan and the price per share
of Common Stock covered by each such outstanding Option, shall be
proportionately adjusted for any increase or decrease in the number of issued
shares of Common Stock resulting from a stock split, reverse stock split, stock
dividend, combination or reclassification of the Common Stock, or any other
increase or decrease in the number of issued shares of Common Stock effected
without receipt of consideration by the Company; provided, however, that
conversion of any convertible securities of the Company shall not be deemed to
have been "effected without receipt of consideration." Such adjustment shall be
made by the Board, whose determination in that respect shall be final, binding
and conclusive. Except as expressly provided herein, no issuance by the Company
of shares of stock of any class, or securities convertible into shares of stock
of any class, shall affect, and no adjustment by reason thereof shall be made
with respect to, the number or price of shares of Common Stock subject to an
Option.

               (b) Corporate Transactions. In the event of the proposed
dissolution or liquidation of the Company, the Board shall notify the Optionee
at least fifteen (15) days prior to such proposed action. To the extent it has
not been previously exercised, the Option will terminate immediately prior to
the consummation of such proposed action. In the event of a proposed sale of all
or substantially all of the assets of the Company, or the merger of the Company
with or into another corporation, the Option may be assumed or an equivalent
option may be substituted by such successor corporation or a parent or
subsidiary of such successor corporation. If the successor corporation does not
agree to assume the Option or substitute an equivalent option, the Board shall
notify the Optionee at least fifteen (15) days prior to the consummation of the
transaction. To the extent it has not been previously exercised, the Option will
terminate immediately prior to the consummation of such transaction.

        14. Time of Granting Options. The date of grant of an Option shall, for
all purposes, be the date on which the Administrator makes the determination
granting such Option, or such other date as is determined by the Board. Notice
of the determination shall be given to each Employee or Consultant to whom an
Option is so granted within a reasonable time after the date of such grant.

        15. Amendment and Termination of the Plan.

               (a) Amendment and Termination. The Board may amend or terminate
the Plan from time to time in such respects as the Board may deem advisable;
provided that, the following revisions or amendments shall require approval of
the shareholders of the Company in the manner described in Section 19 of the
Plan:

                    (i) any increase in the number of Shares subject to the
Plan, other than an adjustment under Section 13 of the Plan;

                    (ii) any change in the designation of the class of persons
eligible to be granted Options;

                    (iii) any change in the limitation on grants to employees as
described in Section 8 of the Plan or other changes which would require
shareholder approval to qualify options granted hereunder as performance-based
compensation under Section 162(m) of the Code; or

                                      -8-
<PAGE>   9

                    (iv) any revision or amendment requiring shareholder
approval in order to preserve the qualification of the Plan under Rule 16b-3.

               (b) Shareholder Approval. If any amendment requiring shareholder
approval under Section 16(a) of the Plan is made subsequent to the first
registration of any class of equity securities by the Company under Section 12
of the Exchange Act, such shareholder approval shall be solicited as described
in Section 19 of the Plan.

               (c) Effect of Amendment or Termination. Any such amendment or
termination of the Plan shall not affect Options already granted and such
Options shall remain in full force and effect as if this Plan had not been
amended or terminated, unless mutually agreed otherwise between the Optionee and
the Board, which agreement must be in writing and signed by the Optionee and the
Company.

        16. Conditions Upon Issuance of Shares. Shares shall not be issued
pursuant to the exercise of an Option unless the exercise of such Option and the
issuance and delivery of such Shares pursuant thereto shall comply with all
relevant provisions of law, including, without limitation, the Securities Act of
1933, as amended, the Exchange Act, the rules and regulations promulgated
thereunder, and the requirements of any stock exchange upon which the Shares may
then be listed, and shall be further subject to the approval of counsel for the
Company with respect to such compliance.

               As a condition to the exercise of an Option, the Company may
require the person exercising such Option to represent and warrant at the time
of any such exercise that the Shares are being purchased only for investment and
without any present intention to sell or distribute such Shares if, in the
opinion of counsel for the Company, such a representation is required by any of
the aforementioned relevant provisions of law.

        17. Reservation of Shares. The Company, during the term of this Plan,
will at all times reserve and keep available such number of Shares as shall be
sufficient to satisfy the requirements of the Plan.

               The inability of the Company to obtain authority from any
regulatory body having jurisdiction, which authority is deemed by the Company's
counsel to be necessary to the lawful issuance and sale of any Shares hereunder,
shall relieve the Company of any liability in respect of the failure to issue or
sell such Shares as to which such requisite authority shall not have been
obtained.

        18. Agreements. Options shall be evidenced by written agreements in such
form as the Board shall approve from time to time.

        19. Shareholder Approval.

               (a) Continuance of the Plan shall be subject to approval by the
shareholders of the Company within twelve (12) months before or after the date
the Plan is adopted. Such shareholder approval shall be obtained in the degree
and manner required under applicable state and federal law and the rules of any
stock exchange upon which the Shares are listed.

               (b) In the event that the Company registers any class of equity
securities pursuant to Section 12 of the Exchange Act, any required approval of
the shareholders of the Company obtained after such registration shall be
solicited substantially in accordance with Section 14(a) of the Exchange Act and
the rules and regulations promulgated thereunder.

               (c) If any required approval by the shareholders of the Plan
itself or of any amendment thereto is solicited at any time otherwise than in
the manner described in Section 19(b) hereof, then the Company shall, at or
prior to the first annual meeting of shareholders held subsequent to the later
of (1) the

                                      -9-
<PAGE>   10

first registration of any class of equity securities of the Company under
Section 12 of the Exchange Act or (2) the granting of an Option hereunder to an
officer or director after such registration, do the following:

                    (i) furnish in writing to the holders entitled to vote for
the Plan substantially the same information that would be required (if proxies
to be voted with respect to approval or disapproval of the Plan or amendment
were then being solicited) by the rules and regulations in effect under Section
14(a) of the Exchange Act at the time such information is furnished; and

                    (ii) file with, or mail for filing to, the Securities and
Exchange Commission four copies of the written information referred to in
subsection (i) hereof not later than the date on which such information is first
sent or given to shareholders.

        20. Information to Optionees. The Company shall provide to each Optionee
and individual who acquired shares pursuant to the exercise of an option, during
the period for which such Optionee has one or more Options outstanding, copies
of all annual reports and other information that are provided to all
shareholders of the Company.



                                      -10-
<PAGE>   11

                         1992 ISOCOR STOCK OPTION PLAN

                          NOTICE OF STOCK OPTION GRANT

Optionee's Name and Address:

Optionee

        You have been granted an option to purchase Common Stock of ISOCOR (the
"Company") as follows:


        Board Approval Date:                      Board Approval Date

        Date of Grant (Later of Board
               Approval Date or
               Commencement of
               Employment/Consulting):            Grant Date

        Exercise Price Per Share:                 Exercise Price

        Total Number of Shares Granted:           No of Shares

        Total Price of Shares Granted:            Total  Exercise    Price

        Type of Option:                           No Shares ISO Shares Incentive
                                                  Stock Option No Shares NSO
                                                  Shares Nonstatutory Stock
                                                  Option

        Term/Expiration Date:                     Expir Date

        Vesting Commencement Date:                Vesting Start Date

        Vesting Schedule:                         Vesting

        Termination Period:                       Option may be exercised for a
                                                  period of 45 days after
                                                  termination of employment or
                                                  consulting relationship except
                                                  as set out in Sections 7 and 8
                                                  of the Stock Option Agreement
                                                  (but in no event later than
                                                  the Expiration Date).

        By your signature and the signature of the Company's representative
below, you and the Company agree that this Option is granted under and governed
by the terms and conditions of the ISOCOR 1992 Stock Option Plan and the Stock
Option Agreement, all of which are attached and made a part of this document.

OPTIONEE:                                         ISOCOR

                                                  By:
----------------------------------------             ---------------------------
Signature

                                                  Title:
----------------------------------------                ------------------------
Print Name

<PAGE>   12




                                     ISOCOR

                             STOCK OPTION AGREEMENT



        1. GRANT OF OPTION. ISOCOR, a California corporation (the "Company"),
hereby grants to the Optionee named in the Notice of Stock Option Grant attached
to this Agreement ("Optionee"), an option (the "Option") to purchase the total
number of shares of Common Stock (the "Shares") set forth in the Notice of Stock
Option Grant, at the exercise price per share set forth in the Notice of Stock
Option Grant (the "Exercise Price") subject to the terms, definitions and
provisions of the 1992 Stock Option Plan (the "Plan") adopted by the Company,
which is incorporated in this Agreement by reference. In the event of a conflict
between the terms of the Plan and the terms of this Agreement, the terms of the
Plan shall govern. Unless otherwise defined in this Agreement, the terms used in
this Agreement shall have the meanings defined in the Plan.

               To the extent designated an Incentive Stock Option in the Notice
of Stock Option Grant, this Option is intended to qualify as an Incentive Stock
Option as defined in Section 422 of the Internal Revenue Code of 1986, as
amended (the "Code") and, to the extent not so designated, this Option is
intended to be a Nonstatutory Stock Option.

        2. EXERCISE OF OPTION. This Option shall be exercisable during its term
in accordance with the Vesting Schedule set out in the Notice of Stock Option
Grant and with the provisions of Sections 9 and 10 of the Plan as follows:

               (a) RIGHT TO EXERCISE.

                    (i) This Option may not be exercised for a fraction of a
share.

                    (ii) In the event of Optionee's death, disability or other
termination of employment, the exercisability of the Option is governed by
Sections 6, 7 and 8 below, subject to the limitations contained in paragraphs
(iii) and (iv) below.

                    (iii) In no event may this Option be exercised after the
date of expiration of the term of this Option as set forth in the Notice of
Stock Option Grant.

                    (iv) If designated an Incentive Stock Option in the Notice
of Stock Option Grant, in the event that the Shares subject to this Option (and
all other Incentive Stock Options granted to Optionee by the Company or any
Parent or Subsidiary) that vest in any calendar year have an aggregate fair
market value (determined for each Share as of the Date of Grant of the option
covering such Share) in excess of $100,000, the Shares in excess of $100,000
shall be treated as subject to a Nonstatutory Stock Option, in accordance with
Section 5 of the Plan.

               (b) METHOD OF EXERCISE.

                    (i) This Option shall be exercisable by delivering to the
Company a written notice of exercise (in the form attached as Exhibit A) which
shall state the election to exercise the Option, the number of Shares in respect
of which the Option is being exercised, and such other representations and
agreements as to the holder's investment intent with respect to such Shares of
Common Stock as may be required by the Company pursuant to the provisions of the
Plan. Such written notice shall be signed by Optionee and shall be delivered in
person or by certified mail to the Stock Option Administrator of the Company.
The written notice shall be accompanied by payment of the exercise price. This
Option shall be

<PAGE>   13

deemed to be exercised upon receipt by the Company of such written notice
accompanied by the exercise price.

                    (ii) As a condition to the exercise of this Option, Optionee
agrees to make adequate provision for federal, state or other tax withholding
obligations, if any, which arise upon the exercise of the Option or disposition
of Shares, whether by withholding, direct payment to the Company, or otherwise.

                    (iii) No Shares will be issued pursuant to the exercise of
an Option unless such issuance and such exercise shall comply with all relevant
provisions of law and the requirements of any stock exchange upon which the
Shares may then be listed. Assuming such compliance, for income tax purposes the
Shares shall be considered transferred to Optionee on the date on which the
Option is exercised with respect to such Shares.

        3. OPTIONEE'S REPRESENTATIONS. In the event the Shares purchasable
pursuant to the exercise of this Option have not been registered under the
Securities Act of 1933, as amended (the "Securities Act"), at the time this
Option is exercised, Optionee shall, if required by the Company, concurrently
with the exercise of all or any portion of this Option, deliver to the Company
an investment representation statement in customary form, a copy of which is
available for Optionee's review from the Company upon request.

        4. METHOD OF PAYMENT. Payment of the exercise price shall be by any of
the following, or a combination of the following, at the election of Optionee:
(a) cash; (b) check; (c) surrender of other shares of Common Stock of the
Company that (i) either have been owned by Optionee for more than six (6) months
on the date of surrender or were not acquired, directly or indirectly, from the
Company, or (ii) have a Fair Market Value on the date of surrender equal to the
aggregate exercise price of the Shares as to which said Option shall be
exercised; or (d) if there is a public market for the Shares and they are
registered under the Securities Act, delivery of a properly executed exercise
notice together with irrevocable instructions to a broker to deliver promptly to
the Company the amount of sale or loan proceeds required to pay the exercise
price.

        5. RESTRICTIONS ON EXERCISE. This Option may not be exercised until such
time as the Plan has been approved by the shareholders of the Company, or if the
issuance of such Shares upon such exercise or the method of payment of
consideration for such shares would constitute a violation of any applicable
federal or state securities or other law or regulation, including any rule under
Part 207 of Title 12 of the Code of Federal Regulations ("Regulation G") as
promulgated by the Federal Reserve Board. As a condition to the exercise of this
Option, the Company may require Optionee to make any representation and warranty
to the Company as may be required by any applicable law or regulation.

        6. TERMINATION OF RELATIONSHIP. In the event of termination of
Optionee's Continuous Status as an Employee or termination of Optionee's
consulting relationship with the Company, Optionee may, to the extent otherwise
so entitled at the date of such termination (the "Termination Date"), exercise
this Option during the Termination Period set out in the Notice of Stock Option
Grant. To the extent that Optionee was not entitled to exercise this Option at
the date of such termination, or if Optionee does not exercise this Option
within the time specified in the Notice of Stock Option Grant, the Option shall
terminate.

        7. DISABILITY OF OPTIONEE. Notwithstanding the provisions of Section 6
above, in the event of termination of Optionee's Continuous Status as an
Employee or termination of Optionee's consulting relationship with the Company
as a result of total and permanent disability (as defined in Section 22(e)(3) of
the Code), Optionee may, but only within twelve (12) months from the date of
termination of employment or consultancy (but in no event later than the date of
expiration of the term of this Option as set forth in Section 10 below),
exercise the Option to the extent otherwise so entitled at the date of such
termination. To the extent that Optionee was not entitled to exercise the Option
at the date of termination, or if Optionee does not

                                      -2-
<PAGE>   14

exercise such Option (to the extent otherwise so entitled) within the time
specified in this Agreement, the Option shall terminate.

        8. DEATH OF OPTIONEE. In the event of the death of Optionee, the Option
may be exercised, at any time within six (6) months following the date of death
(but in no event later than the date of expiration of the term of this Option as
set forth in Section 10 below), by Optionee's estate or by a person who acquired
the right to exercise the Option by bequest or inheritance, but only to the
extent of the right to exercise that had accrued at the date of death.

        9. NON-TRANSFERABILITY OF OPTION. This Option may not be transferred in
any manner otherwise than by will or by the laws of descent or distribution. The
designation of a beneficiary does not constitute a transfer. An Option may be
exercised during the lifetime of Optionee only by Optionee or a transferee
permitted by this section. The terms of this Option shall be binding upon the
executors, administrators, heirs, successors and assigns of Optionee.

        10. TERM OF OPTION. This Option may be exercised only within the term
set out in the Notice of Stock Option Grant, and may be exercised during such
term only in accordance with the Plan and the terms of this Option.

        11. NO ADDITIONAL EMPLOYMENT RIGHTS. Optionee understands and agrees
that the vesting of Shares pursuant to the Vesting Schedule is earned only by
continuing as an Employee or Consultant at the will of the Company (not through
the act of being hired, being granted this Option or acquiring Shares under this
Agreement). Optionee further acknowledges and agrees that nothing in this
Agreement, nor in the Plan which is incorporated in this Agreement by reference,
shall confer upon Optionee any right with respect to continuation as an Employee
or Consultant with the Company, nor shall it interfere in any way with his or
her right or the Company's right to terminate his or her employment or
consulting relationship at any time, with or without cause.

        12. TAX CONSEQUENCES. Optionee acknowledges that he or she has read the
brief summary set forth below of certain federal tax consequences of exercise of
this Option and disposition of the Shares under the law in effect as of the date
of grant. OPTIONEE UNDERSTANDS THAT THIS SUMMARY IS NECESSARILY INCOMPLETE, AND
THE TAX LAWS AND REGULATIONS ARE SUBJECT TO CHANGE. OPTIONEE SHOULD CONSULT HIS
OR HER OWN TAX ADVISER BEFORE EXERCISING THIS OPTION OR DISPOSING OF THE SHARES.

               (a) EXERCISE OF INCENTIVE STOCK OPTION. If this Option is an
Incentive Stock Option, there will be no regular federal income tax liability
upon the exercise of the Option, although the excess, if any, of the fair market
value of the Shares on the date of exercise over the Exercise Price will be
treated as an item of alternative minimum taxable income for federal tax
purposes and may subject Optionee to the alternative minimum tax in the year of
exercise.

               (b) EXERCISE OF NONSTATUTORY STOCK OPTION. If this Option does
not qualify as an Incentive Stock Option, Optionee may incur regular federal
income tax liability upon the exercise of the Option. Optionee will be treated
as having received compensation income (taxable at ordinary income tax rates)
equal to the excess, if any, of the fair market value of the Shares on the date
of exercise over the exercise price. In addition, if Optionee is an employee of
the Company, the Company will be required to withhold from Optionee's
compensation or collect from Optionee and pay to the applicable taxing
authorities an amount equal to a percentage of this compensation income at the
time of exercise.

               (c) DISPOSITION OF SHARES. If this Option is an Incentive Stock
Option and if Shares transferred pursuant to the Option are held for more than
one year after exercise and more than two years after the Date of Grant, any
gain realized on disposition of the Shares will be treated as long-term

                                      -3-
<PAGE>   15

capital gain for federal income tax purposes. If Shares purchased under an
Incentive Stock Option are disposed of before the end of either of such two
holding periods, then any gain realized on such disposition will be treated as
compensation income (taxable at ordinary income rates) to the extent of the
excess, if any, of the lesser of (i) the fair market value of the Shares on the
date of exercise, or (ii) the sales proceeds, over the exercise price. If this
Option is a Nonstatutory Stock Option, then gain realized on the disposition of
Shares will be treated as long-term or short-term capital gain depending on
whether or not the disposition occurs more than one year after the exercise
date. In the case of either an Incentive Stock Option or a Nonstatutory Stock
Option, the long-term capital gain will be taxed for federal income tax and
alternative minimum tax purposes at a maximum rate of 28% if the Shares are held
more than one year but less than 18 months after exercise and at 20% if the
Shares are held more than 18 months after exercise.

               (d) NOTICE OF DISQUALIFYING DISPOSITION. If the Option granted to
Optionee in this Agreement is an Incentive Stock Option, and if Optionee sells
or otherwise disposes of any of the Shares acquired pursuant to the Incentive
Stock Option on or before the later of (i) the date two years after the Date of
Grant, or (ii) the date one year after transfer of such Shares to Optionee upon
exercise of the Incentive Stock Option, Optionee shall notify the Company in
writing within thirty (30) days after the date of any such disposition. Optionee
agrees that Optionee may be subject to income tax withholding by the Company on
the compensation income recognized by Optionee from the early disposition by
payment in cash or out of the current earnings paid to Optionee.

        13. DEFINITION OF CHANGE OF CONTROL. "Change of Control" shall mean the
occurrence of any of the following events:

                    (i) OWNERSHIP. Any "person" (as such term is used in
Sections 13(d) and 14(d) of the Securities Exchange Act of 1934, as amended (the
"Exchange Act"), is or becomes the "beneficial owner" (as defined in Rule 13d-3
under the Exchange Act), directly or indirectly, of securities of the Company
representing 50% or more of the total voting power represented by the Company's
then outstanding voting securities; or

                    (ii) MERGER/SALE OF ASSETS. The stockholders of the Company
approve a merger or consolidation of the Company with any other corporation,
other than a merger or consolidation which would result in the voting securities
of the Company outstanding immediately prior thereto continuing to represent
(either by remaining outstanding or by being converted into voting securities of
the surviving entity) at least 50% of the total voting power represented by the
voting securities of the Company or such surviving entity outstanding
immediately after such merger or consolidation, or the shareholders of the
Company approve a plan of complete liquidation of the Company or an agreement
for the sale or disposition by the Company of all or substantially all of the
Company's assets.

        14. SIGNATURE. This Stock Option Agreement shall be deemed executed by
the Company and Optionee upon execution by such parties of the Notice of Stock
Option Grant attached to this Stock Option Agreement.

                  [Remainder of page left intentionally blank]



                                      -4-
<PAGE>   16


                                    EXHIBIT A

                               NOTICE OF EXERCISE

To:            ISOCOR

Attn:          Stock Option Administrator

Subject:       Notice of Intention to Exercise Stock Option

        This is official notice that the undersigned ("Optionee") intends to
exercise Optionee's option to purchase __________ shares of ISOCOR Common Stock,
under and pursuant to the Company's 1992 Stock Option Plan and the Notice of
Stock Option Grant and Stock Option Agreement dated ___________, as follows:

                      Date of Grant:
                                                  ------------------------------
                      Number of Shares:
                                                  ------------------------------
                      Purchase Price:
                                                  ------------------------------
                      Method of Payment
                      of Purchase Price:
                                                  ------------------------------
               Social Security No.:
                                                  ------------------------------
               The shares should be issued as follows:

                      Name:
                                        ----------------------------------------
                      Address:
                                        ----------------------------------------

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

                                        ----------------------------------------
                      Signed:
                                        ----------------------------------------
                      Date:
                                        ----------------------------------------