Yeah, I got this again today (09/09/2019), and have in the past. Super Monday morning hassle!
I think the last time it happened couple months ago, I downloaded a new Thunderbird email system to fix it? May have to do that again!?
"Sending of the message failed.
The message could not be sent because the connection to Outgoing server (SMTP) smtp.comcast.net timed out. Try again."