So, as of a few days ago, I've got this going on:
"An error occurred while sending mail. The mail server responded: Request failed; Mailbox unavailable. Please check the message and try again."
Suddenly can't send messages from Thunderbird. I'm not the only one. Same problem described here
at the Mozillazine forum, and here
at a Mozilla tech support board. I have not been able to join either board to participate (the latter because the registration page is in Italian). The latter does seem to point to a cause that applies to me. If so, I'm apparently left with a choice of choosing a new email client or choosing a new Reply-to name.
Clearly I'm more griping than soliciting advice, but suggestions are still welcome.