Who is the provider? Call them and ask what is wrong with their server.

I also have this problem. I called AT&T and they said nothing is wrong with their server. It only affects outgoing email. AT&T said it must be Mozilla's server, which made no sense to me at all. All settings in Tbird are correct. I was fiddling around with changing outgoing settings and I managed to get one test email out, but that's all.

Too bad the AT&T person is not smart enough to know that there are no Thunderbird servers. Besides it is THEIR server that issued the error message. Thunderbird is email client software running on your computer.

I managed to get a second email out. I have to play with changing outbound server and port settings. If I change them enough, I guess it confuses their server and the email sneaks out while it's scratching its head. Inbound emails are not affected at all.

http://downdetector.com/status/yahoo-mail

Looks like YAHOO cleaned up their act--sending emails now working. Thanks for the great responses DB

If you read the comments on this site people are having problems sending from their phones and iPads. Thunderbird does not run on those devices. http://downdetector.com/status/yahoo-mail/news/150390-problems-at-yahoo-mail-2