https://support.mozilla.org/kb/cannot-send-messages

I don't see an answer from christ1

He posted a link to an article listing standard faultfinding steps. You need to hit "reply" to be brought back to this discussion.

If Verizon is your ISP, that's your problem. On about Nov 16, 2016, Verizon decided to "enhance security and prevent phishing" by refusing to allow any non-local "From" address, i.e. one that doesn't say "Verizon". They've gotten a storm of howls, and are reconsidering, they say. (I now see that your problem was in August, so I guess you had a different cause for your problem.) For now, I've set Thunderbird aside and am using webmail from the company that hosts my domain.