I had the email server on OS X 10.3.5 die on me. Checking the log, I found this under IMAP (line breaks added):
Sep 17 11:01:19 digitallifestylecenter imapd[3035]: Fatal error:
can't initialize mboxlist environment
Sep 17 11:29:18 localhost imapd[449]: DBERROR db4: fatal region
error detected; run recovery
Sep 17 11:29:18 localhost imapd[449]: DBERROR: dbenv->open '/var/imap/db'
failed: DB_RUNRECOVERY: Fatal error, run database recovery
Sep 17 11:29:18 localhost imapd[449]: Fatal error: can't
initialize mboxlist environment
I had a hard time finding any reference to it, until I found (via Google) this blog entry. It contains a link to this thorough discussion on Apple's Discussion site, which contains a complete walk through on repairing this issue and recovering your mailboxes.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2004091712380234