How can courier-imap and courier-pop accept both username and username@domain?

I have courier-imap and courier-pop installed on a server, and they authenticate with username and password. The user database is LDAP, although, as far as I can see, courier doesn't know that; apparently it uses PAM, and PAM is in turn configured to use LDAP.

I want courier to accept username@domain as an alias for username, but I cannot find how to do it.

(The reason I want it is that the whole service will move to another service which serves many domains, and thus needs the @domain suffix; there will be a transition period when the old server will still be serving the emails, but during which users will be asked to modify their settings from username to username@domain, so the system must be able to work with both.)