I just had a customer contact me asking if I had any idea why his email was being rejected by the Yahoo mailservers (which also deal with all BT addresses).
The rejection message said:
554 Message not allowed –  (in reply to end of DATA command)
A quick Google search, and a look at the message that was rejected, told me that the time on the sending machine was wrong – it was set 1 month in the future. It seems Yahoo won’t accept mail sent from the future, so if you are getting similar rejection messages check your system clock!
I hope this helps someone.