I see what you mean, but I ran into a small mail routing problem that I feel I "hacked" around. There's probably a more elegant and correct solution.
Zimbra: mail02.mydomain.com
Sendmail: mail.mydomain.com
MX-1 mail02.mydomain.com
MX-2 mail.mydomain.com (and this can probably be removed)
User1@mydomain.com = sendmail user
User2@mydomain.com = Zimbra user
user2 has a .forward file in sendmail that is
User2@mail02.mydomain.com. Then in Zimbra I created mail02.mydomain.com as another Zimbra domain and added a
User2@mail02.mydomain.com alias for
User2@mydomain.com. The reason was that sendmail users were not able to send mail to the Zimbra users.
I didn't want to delete the sendmail user accounts and thats what was causing the problem. Would it be possible to have the .forward file say:
User2@mydomain.com? Seemed to me that sendmail would try sending to itself and it might create a routing loop. Ideas on that?