I think an RDBMS is perfect for smaller sites of up to a few thousand users. After that (when you start requiring clustering to support the load and/or storage requirements) it is probably much simpler to just use a big SAN storing Maildirs with as many IMAP and web front-end boxes as you need. But you would lose the nice advantages a pure SQL solution gives you, like precise sorting and (full text) search.As an alternative to all the Java-in-the-database stuff Jin was doing, you could simply write a tiny C program that sits in the .qmail file and POSTs the RFC822 data (and SMTP to/from/helo) to the webserver for instant processing. If it bombs out for whatever reason, qmail will just try again later. (Like Jin said in his article, that problem has been solved)
I once envissioned a mailing list system that worked this way (all the current ones are so 1990) but never got a chance to implement it, like so many - in my mind - good ideas...
-- Bas Scheffers, February 27, 2006