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                                                                     EXHIBIT 5.1

                            [VENTURE LAW GROUP LOGO]



                                November 5, 1999


ISOCOR
3420 Ocean Park Boulevard
Santa Monica, CA  90405

        REGISTRATION STATEMENT ON FORM S-8

Ladies and Gentlemen:

        We have examined the Registration Statement on Form S-8 to be filed by
you with the Securities and Exchange Commission on or about November 8, 1999
(the "Registration Statement") in connection with the registration under the
Securities Act of 1933, as amended, of shares of your Common Stock (the
"Shares") reserved for issuance under the 1996 Employee Stock Purchase Plan and
1999 Stock Option Plan (the "Plans"). 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 and issuance of the Shares under the Plan.

        It is our opinion that, when issued and sold in the manner referred to
in the Plans and pursuant to the respective agreement which accompanies each
grant under the Plans, the Shares will be legally and validly issued, fully paid
and nonassessable.

        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 it appears in the
Registration Statement and any amendments to it.

                                            Sincerely,


                                            VENTURE LAW GROUP
                                            A Professional Corporation

                                            /s/ Venture Law Group

EJB