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                                                                     Exhibit 5.1

                     [Letterhead of Pillsbury Winthrop LLP]

                                 June 15, 2001


Critical Path, Inc.
532 Folsom Street
San Francisco, California 94105

        Re:  Registration Statement on Form S-8

Ladies and Gentlemen:

        With reference to the Registration Statement on Form S-8 to be filed
by Critical Path, Inc., a California corporation (the "Company"), with the
Securities and Exchange Commission under the Securities Act of 1933, as amended,
relating to 16,741,348 shares of the Company's Common Stock (the "Shares")
issuable pursuant to the Amended and Restated 1998 Stock Plan, 1999 Stock Option
Plan and 1999 Employee Stock Purchase Plan (the "Plans"), it is our opinion that
the Shares have been duly authorized and, when issued and sold in accordance
with the Plans, will be validly issued, fully paid and nonassessable.

        We hereby consent to the filing of this opinion with the Securities and
Exchange Commission as Exhibit 5.1 to the Registration Statement.

                                          Very truly yours,


                                          /s/ PILLSBURY WINTHROP LLP