I now use Mozilla. Just 2 of my regular contacts have btinernet.com e-mail addresses. they receive my e-mails ok, but I can't get teir replies

My guess is that your email provider is using a blacklist that has decided that bt.com is not trustworthy, and so blocks their messages to you. But you'd hope that it would appear in your spam or bulk mail folders or that you'd get a notification. There is no "mozilla server" involved here. I have a bt address myself, so we could test my theory if you write something to me at: xenos at btinternet dot com I can reply (if this wretched account still works!) from two different accounts (i.e. a bt account and a non-bt account) so we can check out the full there-and-back round loop too.