I run a postfix, amavis-new, dovecot setup for mail service on my LAN, which works nicely for a long time. I recently added a horde imp server in my DMZ - actually behind IPv4 NAT using port forwarding. Which also works quite nice according to my early tests.
However, if I write e-mails using imp, these are classified as UBE dropping me a mail note: "Considered UNSOLICITED BULK EMAIL, apparently from you"
Analyzing the header it seems that imp acts as an MTA. I see two "Received:" headers: Smartphone->Horde-Imp, Horde-Imp->Postfix. I had expected Imp to act as MUA instead, but ...
The mail.log shows that amavis marks the mail as SPAM. But there's no log entry as to why it deems it such. Amavis is configured to use Razor and Pyzor. I did another test where the client had a different IP and the mail was actually delivered.