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Apologies for the delay, I wanted to take some time to think about it overnight. Nothing came to mind, so I contacted BT today. Looking at the email addresses of the three contacts I was sure about, I noticed that they were all gmail. BT reminded me where to find the Blocked Senders list (I'd forgotten, but have now made a note of it), and there at the top of the list was gmail. I have no idea how it happened; I type very quickly, so possibly I caught the wrong key by mistake. Anyway, all is now well. Many thanks for your advice, youngmafbog and Buenchico.