Non-caching IMAP/SMTP proxy for ClamAV

Hello. I hope someone has solved this problem before.

I'm trying to setup an imap/smtp proxy for a small LAN, where most of the devices on the LAN are Android/Apple mobiles. The sole purpose of the proxy is so ClamAv jail can scan in-flight e-mail traffic & drop if positive.

Solution Requirements:
* Transparent, Lightweight, Non-Caching.
* Support for TLS, connection. Preference: query & close (not keep-alive).
* Handoff to ClamAV, then process message based on scan result.
* User account credentials not available to sysadmin (LDAP ?) or preferably, credentials on device directly passed to main server (ex: gmail) so that no user account maintenance is required.
* POP3 is not used (an IMAP-only solution is OK).
* Caldav/Carddav support would be nice. These don't need AV scanning, but unsure about directly NATting that traffic?

I found mail/davmail, but looks like it's for MS Exchange server only.
Perhaps there's a much easier solution I'm overlooking?