I have the same problem. Ordinarily, if I give up, save the outgoing mail to Drafts, then send, I can eventually get it to go. No more. I am absolutely STUCK and unable to send email. Message is always the same: "Sending of message failed. The message could not be sent because the connection to the Outgoing server (SMTP) outbound.att.net timed out. Try again." Repeated failure. ALSO, instructions given earlier at this site don't match what pops up on my computer. Windows 11 / Thunderbird 38. I got Tools-Options-Advanced-Config Editor. When I entered timeout in search slot, nothing happened. I scrolled down, found mailnews.tcptimeout and changed from 100 to 1000; no effect, so went back and changed to 100.
Nothing is working. HELP. AT&T says this is a Thunderbird problem. Probably correct, since I have no problems sending gmail.
Thank you, anyone who can help.