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                                                                     EXHIBIT 8.1

                  [Wilson Sonsini Goodrich & Rosati Letterhead]

                                December __, 1999

Critical Path, Inc.
320 First Street
San Francisco, CA 94105

Ladies and Gentlemen:

        We have acted as counsel to Critical Path, Inc., a California
corporation ("Parent") in connection with the proposed merger (the "Merger")
among Parent, Initialize Acquisition Corp., a California corporation and
wholly-owned transitory merger subsidiary of Parent ("Merger Sub"), and ISOCOR,
a California corporation ("Company") pursuant to an Agreement and Plan of
Reorganization dated as of October 20, 1999, (the "Merger Agreement"). The
Merger and certain proposed transactions incident thereto are described in the
Registration Statement on Form S-4 of Parent (the "Registration Statement"),
which includes the Proxy Statement/Prospectus of the Company (the "Proxy
Statement/Prospectus"). This opinion is being rendered pursuant to the
requirements of Item 21(a) of Form S-4 under the Securities Act of 1933, as
amended. Unless otherwise indicated, any capitalized terms used herein and not
otherwise defined have the meaning ascribed to them in the Proxy
Statement/Prospectus.

        In connection with this opinion, we have examined and are familiar with
the Merger Agreement, the Registration Statement, and such other presently
existing documents, records and matters of law as we have deemed necessary or
appropriate for purposes of our opinion. In addition, we have assumed (i) that
the Merger will be consummated in the manner contemplated by the Proxy
Statement/Prospectus and in accordance with the provisions of the Merger
Agreement, (ii) the truth and accuracy of the representations and warranties
made by Parent, Merger Sub and the Company in the Merger Agreement, and (iii)
the truth and accuracy of the certificates of representations to be provided to
us by Company, Parent and Merger Sub.

        Based upon and subject to the foregoing, in our opinion, the discussion
contained in the Registration Statement under the caption "Material United
States Federal Income Tax Consequences of the Merger," subject to the
limitations and qualifications described therein, sets forth the material United
States Federal income tax considerations generally


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Critical Path, Inc.
December __, 1999
Page 2

applicable to the Merger. Because this opinion is being delivered prior to the
Effective Time of the Merger, it must be considered prospective and dependent on
future events. There can be no assurance that changes in the law will not take
place which could affect the United States Federal income tax consequences of
the Merger or that contrary positions may not be taken by the Internal Revenue
Service.

        This opinion is furnished to you solely for use in connection with the
Registration Statement. We hereby consent to the filing of this opinion as
Exhibit 8.1 to the Registration Statement. We also consent to the reference to
our firm name wherever appearing in the Registration Statement with respect to
the discussion of the material federal income tax consequences of the Merger,
including the Proxy Statement/Prospectus constituting a part thereof, and any
amendment thereto. In giving this consent, we do not thereby admit that we are
in the category of persons whose consent is required under Section 7 of the
Securities Act of 1933, as amended, or the rules and regulations of the
Securities and Exchange Commission thereunder, nor do we thereby admit that we
are experts with respect to any part of such Registration Statement within the
meaning of the term "experts" as used in the Securities Act of 1933, as amended,
or the rules and regulations of the Securities and Exchange Commission
thereunder.

                                            Very truly yours,

                                            WILSON SONSINI GOODRICH & ROSATI
                                            Professional Corporation
