This is not really a question because I have solved it, but just a mixture of amazement and frustration.
<postmaster@btinternet.com> sent me an email explaining all the reasons that an email that was sent by me was rejected by gmail. Guess what ? They put their own email straight in my spam folder.
To fix this I had to click on the email in my spam folder and set it as "Safe Sender" !!!
So <postmaster@btinternet.com> is a source of spam ? It took me a long time to believe this was the problem.
