Thanks very much for your reply. I have an older machine running claws-mail 3.8.1 and a newer machine running 3.11.1. Some but not all correspondents stopped being able to receive messages from my 3.8.1 machine, while still hearing from 3.11.1. I changed the 3.8.1 "sent" configuration (which had previously used a different server), to be the same as the 3.11.1 settings, including the use of the 587 port and TLS. These in turn were based on documentation provided by the server.
You talk about "running encrypted" vs unencrypted. Is this something I can control using the "preferences for current account"? I had one specific worry. The failure to send on 3.8.1 was accompanied by the message > Warning SSL connection failed (A TLS packet with > unexpected length was received) In my ignorance, I thought this might mean malware was adding something to my messages.
