Well, that was a convenient accidental necro, as I've had this very same error occur.
I've contacted my ISP to see if I can get firewalling disabled on my line entirely and just go back to using my router/modem's firewall.
I'm not at all knowledgeable about firewalls, so here's my configuration's ruleset:
Inbound:
permit tcp any host mail.sonic.net eq 25
deny tcp any any eq 25
deny tcp any eq 25 any
permit ip any any
Outbound:
permit ip any any
My computer is in DMZ+ mode on my router's firewall, so that's not the issue.
Any clue if that ruleset would block Trem, or is there another issue?