Add this SMTP server to your contact list?
Instant messenger has proven relatively immune to the scourge of spam that affects email today. Tagging messages from those on your buddy list, you can implement a system similar (in theory) to SPF, prioritizing mail from your buddies.
For an extra layer of authentication, write a plugin for the instant messenger that communicates "i've sent you message-id foo", to confirm that the email is indeed from a known contact. At this point anything with a From: address that matches a contact, but a Message-ID: for which no notification was received, is automatically trapped as questionable and held for review (unless, at some point, the Message-ID: is received from the sender in question).