Thanks Cory -- I have been slow with an update but FWIW I am on Mac OS 10.14, still using Mac Mail and with only one Mail account (IMAP).
I live in Portugal and the ISP is called Novis. The problem has now gone away in a most interesting fashion which may help others.
I use Nord VPN and every few weeks I vary the country that is seen as being the source of my IP address. A day or two ago I tried to buy something from Amazon. I was told that what I was asking for could not be sent to Finland. Finland? Something clicked. I had been connected to Finland by Nord for about three weeks - I quickly pulled up the selector screen for the VPN and switched to Portugal. Immediately there was a Whoosh !! Sound of mail departing. Problem solved. But why? It would be good to Know! I've been connected to Poland, Lithuania, Latvia, Czech republic, Germany France and Ireland, all without the slightest problem.