1
                                                                     Exhibit 5.1


                                 April 11, 2000





Quintus Corporation
47212 Mission Falls Court
Fremont, CA 94539

Ladies and Gentlemen:

     We have acted as special counsel to Quintus Corporation ("Quintus") in
connection with its Registration Statement on Form S-4 (File No. 333-33422) (the
"Registration Statement") and Amendment No. 1 to the Registration Statement,
filed with the Securities and Exchange Commission under the Securities Act of
1933, as amended (the "Securities Act"), relating to the registration by Quintus
of shares (the "Shares") of its common stock, par value $0.001 per share, to be
issued pursuant to the terms of the Agreement and Plan of Merger dated as of
February 25, 2000 between Quintus and Mustang.com, Inc. (the "Merger
Agreement").

     We have examined originals or copies, certified or otherwise identified to
our satisfaction, of such documents, corporate records, certificates and other
instruments, and have conducted such other investigations as we have deemed
necessary or advisable for the purposes of this opinion.

     On the basis of the foregoing, we are of the opinion that the Shares have
been duly authorized and, when issued and delivered in accordance with the terms
and conditions of the Merger Agreement, will be validly issued, fully paid and
non-assessable.

     We are members of the Bar of the State of New York and the foregoing
opinion is limited to the laws of the State of New York and the General
Corporation Law of the State of Delaware.

   2

Quintus Corporation                     2                         April 11, 2000

     We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to us under the caption "Legal
Matters" in the Proxy Statement/Prospectus constituting a part of the
Registration Statement.


                                       Very truly yours,

                                       /s/ DAVIS POLK & WARDWELL
                                       ----------------------------------