The layout of courier sources has changed. So we have a courier-authlib now, providing authentification to other courier services. Thus courier-imap depends on courier-authlib and the configuration of the authentification layer has moved from /etc/courier-imap to /etc/authlib.
Moreover, the courier family depends on the Berkeley db package, which has undergone an API change recently and thus is in testing. Therefore the new courier packages will be available from testing only until db-4.3.x moves to extra.
Additionally, courier-mta is in testing and will move to extra when time has come. This mailserver provides SMTP, POP3, IMAP, Webmail etc. and authentificates all this against one source, configured in courier-authlib, without the pain of tricky configurations.
All this is new and just roughly tested. If you find some quirks, bugs or have suggestions, go bother him. He'll let me know.