I got this too with an account I was looking after and the problem was that, despite having changed the email address on the account, it still insisted on emails coming from the address that was originally registered. You'll probably have to phone them, which is a PITA. Come December I will at last be free of Fasthosts, whom I am now paying monthly to do absoultely nothing (but that's another story!).
Kevin
Plusnet Value
Using OpenDNS