One technique for preventing spammers hosing your SMTP server is to set your lowest MX record to something fake.
I'm having an issue with Postfix not being able to connect to the Meaningful Use Transport Testing Tool.
Apr 7 12:03:11 myserver postfix/smtp: 1F119B60AEC: to=, relay=none, delay=265111, delays=265110/0.06/0/0, dsn=4.4.1, status=deferred (connect to transport-testing.dnsops.gov[188.8.131.52]:25: Connection refused)
I can send emails to the TTT from Outlook or Gmail, so I know that much works. Does Postfix try to hit the other MX records if it gets a connection refused?