
<PAGE>

                                                                     EXHIBIT 5.1


                 [Wilson Sonsini Goodrich & Rosati letterhead]


                                 May 26, 1999



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

     Re:  Registration Statement on Form S-1
          ----------------------------------

Ladies and Gentlemen:

     We have examined the Registration Statement on Form S-1 filed by you with
the Securities and Exchange Commission on May 11, 1999 (the "Registration
Statement"), in connection with the registration under the Securities Act of
1933, as amended, of up to 4,000,000 shares of your Common Stock (the "Shares").
As your legal counsel, we have examined the proceedings taken, and are familiar
with the proceedings proposed to be taken, by you in connection with the sale of
the Shares.

     It is our opinion that the Shares, when sold and issued in accordance with
the Registration Statement, will be legally issued, fully paid and non-
assessable.

     We consent to the use of this opinion as an exhibit to the Registration
Statement and further consent to the use of our name wherever appearing in the
Registration Statement, including the Prospectus constituting a part thereof,
and any amendments thereto.

                                        Very truly yours,

                                        WILSON SONSINI GOODRICH & ROSATI
                                        Professional Corporation

                                        /s/ Wilson Sonsini Goodrich & Rosati
