Open Tools > Options > Security > Passwords > Saved passwords

Find the offending smtp entries (there may be more than one for the same user name) and remove them all. If the list is long, use the search box to narrow it down to entries for the affected account only. Use the username or address of the account as the search term. If you attempt to send, Tbird will prompt for the password and save it afresh.

I appreciate the effort, Stans. But it did not work. There is only one user on this computer, and only one email account. Deleting the one SMTP entry and then trying to resend a text message gave same response, that the "login to account "Verizon Online" failed" followed by "Login to server smtp.verizon.net with username [redacted] failed." going to "retry" immediately returns the failure message. going to "enter new password" and typing it in also gets you back to the failure announcement. Then going to "cancel" gets you to a hung screen, with the message and the "connected to ..." message.

There may an outage with Verizon. Another user is reporting the same issue: https://support.mozilla.org/questions/1283754

I started having the same problem this morning. I called Verizon, talked to a live person, but they said they aren't involved with email anymore, it is all AOL responsibility.

Since this problem started, I have also had one inbound email in Thunderbird display improperly, with the header from one email matched with the body of an unrelated email.

https://downdetector.com/status/aol/

https://jimmytechsupport.com/no-verizon-not-shutting-verizon-net-email-address/

A solution is available in here: https://support.mozilla.org/questions/1283746