Problem found. Well, problems found.
Firstly:
SMTP error from remote mail server after MAIL FROM:<mail@golfgtiforum.co.uk> SIZE=3379:
host mx1.hotmail.com [65.54.188.126]: 550 SC-001 Unfortunately, messages from 70.38.11.38 weren't sent. Please contact your Internet service provider since part of their network is on our block list.
Forum on Hotmail junk email block list.
Secondly:
All the bounce messages filled up the forum mail account. Just clearing that, along with all the email in there from people replying to pm notifications, even though it says "IMPORTANT: Remember, this is just a notification. Please do not reply to this email".