For incoming mail I once had a system wide procmail rule that would
pipe a copy of every incoming mail to something like bzcat >>
/mnt/Archive/username-Incoming-Archive.bz2.

When you bunzip2ed the archive you got a mbox mail file. If a user
needed to go through their archive I just created a decompressed copy
of the file and put it under ~/mail, and they subscribed to it as they
would any other IMAP folder.

I never got around for something similar for outgoing mail.

-- 
Andrew S. Zbikowski | http://andy.zibnet.us
IT Outhouse Blog Thing | http://www.itouthouse.com