You should talk to your email provider.

I haven't found a direct way to do that. Only this forum. I use Thunderbird.

If you did not notice your email addresses are @verizon.net. Verizon is your provider. Thunderbird is email client software not your provider.

Actually, my provider is now AOL. Verizon is no longer in the email business. AOL's support email is down for now (or so they say) and you have to be a paying customer to access their tech support by phone. I am not. So that's why I posted here. Thanks anyway.

It looks to me there's a mismatch between the sending address and the server you're trying to send through. Open Tools/Account Settings, select an account in the left pane, then check the Outgoing Server (SMTP) entry in the lower right pane. Is the account for user name X sending on the smtp server with user name X or the one set up with user name Y? Even though, in this case, both accounts are Verizon accounts, Verizon may require a strict relation between the sender's user name and the outgoing server's user name. The outgoing servers are defined in Tools/Account Settings, under Outgoing Server (SMTP) at the bottom of the left pane.

I added a second outgoing server option with the user name of the nonworking account. The account settings show the default server as being operative for both accounts with my user name. Nevertheless, for reasons I don't understand but will not question, at the moment mail can be sent from her account, so I thank you.

I've been struggling with this one. Maybe my solution will help someone else. I have 3 AOL accounts and there were three identical titles for the SMTP server on the page.

AOL Mail -smtp.aol.com,
AOL Mail - smtp.aol.com,
AOL Mail - smtp.aol.com (default)

By editing those titles (I added relevant initials before AOL on each line) I was able to distinguish which was which to make sure I selected the correct one for which email address. I could then select the correct one for each user in settings and all is now well.