This might happen if you're trying to send from account X through the smtp server of account Y, and mail provider Y now requires that you can send on their server only from account Y. Open Tools/Account Settings, select an account in the left pane, then look at Outgoing Server (SMTP) in the lower right pane. Does the smtp server match the account?

I appreciate the reply, but I've checked and re-checked the settings. When I first set up the account about two years ago the servers were pop.mailserver and smtp.mailserver. Since I posted the question here, I've been researching the problem and the ISP has changed those setting both to mail.mailserver using different ports. They say I need to add an exception to the Security Certificates but Thunderbird has not yet found that server. I've asked them for advice, but haven't heard back.

Thanks again for the response. If you can't help with the exception issue, I'll just mark this as closed.

If you have changed the settings to ones that are (probably) more secure, the error message about adding an exception may be a result of an antivirus program scanning mail connections.

http://kb.mozillazine.org/SSL_Security_Error

If you can post your server settings, along with your AV program, if any, and the exact error message, someone may be able to reply with a solution.

Thanks for the suggestion, but the ISP's configuration instructions says to add the exception. That said, I just tried sending again, and the message went through without the exception being added. I'm beginning to suspect the ISP had an issue which took longer to resolve than it should have.

Thanks for your time and help. No further response needed at this time.