As the message is from their server, the problem is on their end. You may have hit a limit imposed by Yahoo/Frontier. You'll have to ask them about it though.

That was pointed out to them, but they still said that it was from Mozilla/Thunderbird's side. If it isn't, can you give me some more info to ask them? Thank you.

Please post the entire error message.

christ1 said

Please post the entire error message.

"An error occurred while sending mail. The mail server responded: 5.7.1 We were unable to deliver your message. Please try resending your message by adding some text.. Please check the message and try again." The subject line and message both had text. Also, the error message didn't appear until Thunderbird presented the "email sent" box.

See http://kb.mozillazine.org/5.7.1_Unable_to_relay

Presumably you're using your ISPs email service, and they require you to connect to the Internet through them. Is that the case for you?

christ1 said

See http://kb.mozillazine.org/5.7.1_Unable_to_relay Presumably you're using your ISPs email service, and they require you to connect to the Internet through them. Is that the case for you?

Yes, but the only time I have any problem is when my distribution list gets to be about 100 addresses.

Well, as said before, the error message is from their server, and they should be able to tell you what's wrong, or what limit you hit.

Thanks. I'll try again.