What are the recommended settings they gave you? What error messages are you getting? What works and what does not? What may have changed, causing the account to stop working?

I'm experienced in setting up imap and smtp, etc. In fact, the mobile setup on my iPhone works and that is "mail5.hostingplatform.com" and yes, I tried that on Thunderbird too. The one I'm using is the exact same as the incomming and that's "mail.mydomainname.com" along with the correct ports (of course it's not literally "mydomainname" it's the domain for my email). Port 143 on receive and 587 on outgoing. I've tried every security protocol combo possible including none. I'll attach a screenshot of the error message from the server. The only other error was password or authentication related so it seems I'm past that and into the "meat" of the problem with this error:

See attached image.

Thanks.

I wonder if an anti-virus is interfering. Something is causing an incorrect line to be inserted into the message.

I use AVG on my computer and have forever. I don't believe it does anything to anything going out anyway. Now if Thunderbird has a setting for that, please tell me where to look.

Any other ideas? I'm still stuck.

All I can share is that I did locate several others with same problem and all of them use AVG.

It might be the signature that AVG includes. If you can turn that off, it may resolve the issue.

Wow. That was it. Thanks!

I added Thunderbird to the "safe app" list and WHA-LA, the email sent.

Kewl! I'm glad it's working. I'll keep this in mind if I see more problems like this. They just started appearing in last week or so.