Mail Switcher

[Tom Grove]

Does anyone have any ideas on how I would solve the following problem?

I have two mail servers.  One is running cyrus imap and the other 
courier.  I am in the process of moving users over one at a time and 
would like to put some kind of proxy box in front of the mail servers.  
I would like to have the proxy listen on port 143 for connections and 
grab the usernames.  Once this happens I want the users connection 
routed to the appropriate mail server.

Any ideas?

-Tom