Thunderbird - SMTP timeout message on send, but email sent


I’ve read through many of the similar topics on this and they all relate to email not getting sent. I’m getting the following notice from Thunderbird, yet the message is almost always sent. And sometimes I don’t get the message and life is good. The message appears within one second.

Sending of message failed.
The message could not be sent because the connection to SMTP server timed out. Try again or contact your network administrator.

I’m using SSL/TLS on port 465, normal password authentication, my full email address as username. My Tbird TCP timeout is the default 100 (seconds, I assume).

Thanks for any clues.