IMAP is a sophisticated protocol for remote and shared storage of email, and Mulberry is an IMAP client renowned for implementing its side of that protocol fully and with careful adherence to standards. In addition to IMAP features and extensions such as subscribed mailboxes, sorting, threading, access control, quotas, and namespaces, Mulberry also supports either online or disconnected mode, POP3, remote storage of address books and preferences, LDAP, and remote calendaring and scheduling. Mulberry was distributed as a commercial application, first from Cyrusoft International, then from ISAMET, both of which late last year (and both of whose Web sites have closed down). In an astonishing turn of events, however, Mulberry has now been re-released as  by its original developer, Cyrus Daboo.
TidBITS has mentioned Mulberry in the past, but we've never done a full-fledged review, and I'm not about to do one now. (Important disclosure: in late 1996-97 I had a brief but rather central association with the Cyrusoft startup process, but I soon elected to have nothing to do with the project.) Personally, I find just about the whole of Mulberry's interface pretty annoying, but since I'm not an IMAP user, I don't need the features and complexity that Mulberry provides, making me a poor judge of the program. Those who do need a good IMAP client might find Mulberry well worth investigating.
Mulberry is available for Mac OS X 10.3 or later, Windows, and Linux. It is a. Development has officially ceased, so I would not expect Mulberry ever to become a universal binary. But you never know...