Are you on Spectrum/Charter/TWC/roadrunner? Fixing this error may depend on which settings you apply, from the many varieties offered by Spectrum:

https://support.mozilla.org/en-US/questions/1275339

Yes, Spectrum.

As I said here https://support.mozilla.org/en-US/questions/1275339#answer-1276614

Use the Config Editor to set mail.server.default.send_client_info to false (right click the preference in the Config Editor to get the menu of actions.

Basically spectrums server is mis-configured, but as they insist they have had only one person contact them about the issue insist it is a Thunderbird issue.

However Thunderbird has an option to turn off the feature that spectrum's server says it supports and does not so you can workaround their issue. Reporting it to them might be an idae though as they are saying no one is doing it. They can not fix what they don't know is broken

Thanks for your reply. I've already contacted Spectrum with no help. I'm not a computer guru, but how would I disable that feature in Thunderbird? Thanks for your help.

pennypincher said

Thanks for your reply. I've already contacted Spectrum with no help. I'm not a computer guru, but how would I disable that feature in Thunderbird? Thanks for your help.

You would follow the instruction in my last post.

Menu app icon > Options > Options > Advanced > 'General' tab Click on 'Config Editor' button It will say be careful; :) In search type: info look for this line: mail.server.default.send_client_info If the Value says: 'True' double click on that line to toggle the value to 'False' OR Right click on the line and choose 'Toggle'.

Close 'about:config' - top right X

Thanks to both Matt and Toad-Hall for the suggestions. I get nervous and my hand shakes a bit when changing settings that I know nothing about, but this change has made the error message go away. Thanks again.