[Exim] Wanted: encrypted mail storage

Hi folks--

Does anyone know of a way to encrypt the mail storage? My goal is to
add a layer of privacy to the mail--especially mail for virtual
users--which is stored on the server and replicated on the server
backups.

- An ideal method would support per-user/virtual-user keys for
delivered messages.
- Queued and/or frozen messages could use the TLS key.
- Both the MTA(exim) and IMAP server (courier-imap) would need to
support the method chosen.

If it matters, I'm configured for Maildir format.

Thanks,
--rich