OK, somebody else is working on an XDM replacement, so I'll abandon mine.
I've ported part of Dartmouth's BlitzMail system to run on Linux. BlitzMail is a really sweet Mac application with some advanced features. The major components of the system are:
The Dartmouth Name Directory (DND). This is a UNIX-based server which provides authentication services for applications all over campus. You can use one name and password everywhere--for mail, the file server, unlocking screensavers and even Kerberos authentication. The DND server requires POSIX threads for performance reasons, and can handle thousands of users without a problem. The DND server supports several storage subsystems, including flat text files and Oracle databases. This currently runs on my RedHat Linux 5.1 box at home without any problems. Unfortunately, it's a royal pain to set up.
The DND client library and tools. The DND client library provides a convenient interface for validating users and looking up other database information about them (such as file server quotas). A few command-line tools allow you to perform fuzzy DND queries for people's names and edit some of your own DND information. This appears to run fine on my machine, and I've made preliminary RPMS.
The BlitzMail server. The BlitzMail server itself is a truly remarkable creature. It speaks SMTP, POP3 and a custom protocol conceptually similar to IMAP (but much older). The server supports mailing lists, fuzzy alias matching and authenticated senders. Mail is stored on the server and can be accessed from anywhere on campus simply by entering a username and a password. Like the DND server, the BlitzMail server uses pthreads and supports both flat-file and Oracle databases. The best feature of the Blitz server is transparent clustering, where multiple servers divide the work load without user intervention. Thanks to the DND, it's even possible to migrate a user's account from one server to another without requiring the any configuration changes on the client. I'm currently porting this to Linux.
The BlitzMail clients. Wired Magazine chose Dartmouth College as the most wired school in 1998, beating out even such places as MIT. Within twenty-four hours of arriving on campus, 98% of Dartmouth students have sent or received an e-mail message. Without e-mail, you couldn't have a social life at Dartmouth. It's even the preferred method of scheduling parties.
This success is due to three things: complete dorm-room wiring, 100% computer ownership, and the BlitzMail client. The client allows students to walk up to any machine on campus, enter their name and password, and access their e-mail account. Both logins and destination addresses can be specified as real names or nicknames. There's no need to ever remember cryptic abbreviations. The client supports a push-based notification procedure, so new mail arrives instantly.
A client currently exists for the Macintosh, and a Windows version is under development. There's one Motif and two text-based clients for Unix, although none of these have all the features of the official ones.
Somebody with too much free time on their hands could modify Balsa to look like the Mac client and integrate communications code from the Unix clients. Any volunteers?