I posted a separate ticket for this problem - Mozilla forced me to create a problem before I could create an acccount here). Text of that ticket follows:

Yesterday afternoon at approx 3:30 pm (July 22 2017) I successfully sent four (4) emails. I tried to send more at approx 5 pm, received message"

Sending of the message failed. The message could not be sent because the connection to Outgoing server (SMTP) smtp.cox.net timed out. Try again.

I have spoken with my ISP (Cox Communications, Phoenix), they say my account is fine. It is active, I am not blocked from sending emails. Further, I successfully signed in to their online email service, and was able to send emails without error.

Cox says the problem is with my email software, or a setting. Settings were changed to day per Cox recommendations (July 23 2017). That I can identify, nothing changed on my PC yesterday.

Running Windows 10, 64 bit, all maintenance is current.

I have this problem also. Can receive ok, and send from phone app, online at outlook.com and also from Mail (comes with Win10) but not from T/Bird. Have used for many years without a problem. Exhausted Microsoft support ... settings are all ok. The fact that I can send from other sources shows that account is ok.

Something has changed in T/Bird and no-one seems to know about it, believe it or have a solution. But there is a problem here.