I just realized that I had mistakenly left the "authentication" setting turned off (in the mail server settings). I entered the required user name and password, and then my attempt to send a message resulted in 15 or 20 seconds of activity while a notice appeared stating "Message Being Sent". However that process ended with that same "Send Failed...server connection interrupted". So, something still not quite right.
The email account I'm trying to use is Yahoo Mail , and I do not have Yahoo Mail Plus. Is this the reason for my problem? I've had no problems using the Mail app to send/receive Yahoo mail, so I didn't anticipate a problem with the email feature in Files, but...?