I got a Spam email, purportedly from BT, just half an hour ago. Just to make sure, as it's subject line was "non delivery of Sent emails". I rang BT.
After their operator, a very pleasant guy called Andrew, had confirmed that it was Spam. I put this problem to him.
I have no idea what he did but, just a "Log in", then a couple of quick emails and everything seems to be working again.
I am holding my breath!