Email migration from VopMail

[David Babler]

I have a customer who wants to move his dialup customers (600+ accounts)
from VopMail on Windows 2000 to Unix. So far, so good. Unfortunately, he
has all of his customers configured with a non-standard setup that
VopMail supports for virtual domains, though 98% of them are in the real
domain. Specifically he has them all set up to present a "username" as
'user at example.com', the same as their realmed dialup login.

He needs SMTP, ASMTP, POP3 and IMAP. He DOES NOT want to have to have
any customers change their setups or email addresses and I am having a
devil of a time explaining how non-standard VopMail is. Does anybody
know of any way that sendmail or postfix and anybody's POP3 and IMAP can
be forced into authenticating with such a non-standard login?

-Dave