I've been exchanging emails with the tech guys today and they are obviously having a few problems trying to tract this problem down. I think it has something to do with people requesting being notified when replies are made on a thread. This causes the system to try to send mail and that is where the problems lie.
As to what the solution is I really don't know! But I am confident that the tech guys will sort it out soon.
I've just spoken to the Tech Guys and they think it has now been fixed. I have just posted a test message without getting the error so hopefully it should be OK.