[Dovecot] Dovecot username mapping

[Adam "Fuzzy" Kennedy]

Ok, here's my situation. I currently run IMail 7.x with about 80 domains 
and nearly 4000 users.

I want to switch this to a Linux server.

I'm using Virtualmin to manage the users/postfix.

I have dovecot working just peachy with the setup, except for one 
problem. We are an ISP. The easiest thing for us (and our users) is to 
have them use their full email address as their username.

What I need is some way to have Dovecot take a username and map it to 
the real unix user. Our unix users are created as "username-domain.com"

When our users login to our current email system, they enter their 
username as "username at domain.com". I for the life of me cannot figure 
out how to make Dovecot take "username at domain.com" and know that the 
unix username is really "username-domain.com"

Ideas?

-Adam "Fuzzy" Kennedy