Expanding on what Jack said, if you install the Auth_POP3 plugin, then you just specify the domains you want to handle under Setup->Incoming Email->Individual mode. Now, when a user goes to the login prompt and enters a username/password, if that username/pass is a valid POP account, then it lets the user login and uses that POP account as the users mailbox.
edit: detached thread
Cheers,
Alex
--
Gossamer Threads Inc.
edit: detached thread
Cheers,
Alex
--
Gossamer Threads Inc.