I have had similar problems over the past few days and whilst I have now
contacted BT and sorted it - and changing the port to 587 worked, I would
certainly recommend ringing them. If you are with BT, the number is 0845 600
7030. This number is in the back of the Getting Started Booklet and whilst
you will be charged (calls charged at up to 4p a minute on the BT network) -
they will proxy (gain remote access) onto your pc and sort the problem for
you. The lady I have just spoken to, even asked me to test the system whilst
she was still on the phone to make sure I was happy. Hope this helps.