On TB v78 the minimum requirement for SSL has increased and probably your email service provider still uses deprecated security for the mail server. Can you try to set lowest security by following the below steps and let us know how it works? From the menu at the top right, go to Options. Scroll all the way to the bottom and click on Config Editor -> Skip past the warning. Scroll down until you find security.tls.version.min (or paste security.tls.version.min to upper frame) Change the value 3 to 1. Restart TB. If it's still not working you may have to change the preference security.tls.version.enable-deprecated to true

OMG thx a lot mate, but this dont help at all. i still have the same issue here, i even did uninstall it en reinstal and did your settings here,,,, still the same thing about cerftificate here,,, this is so strange,,, runs ok same settings on blue mail, outlook and so on,,,

When I send mail I get an error message that thunderbird can not send mail because the certificate is personally signed?

Please post a screenshot of the error. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Since you can receive messages it's rather unlikely that this is related to a TLS version mismatch. Recommending to mess with Thunderbird TLS security settings without any evidence that there is indeed such a mismatch is bad advice IMV. Make sure you do reset the prefs you modified back to the default values.

I think this is bug 1665577.

It looks like it's fixed for the TB 83 branch, so it might appear in the next point release of 78.

It seems to not be present in 68, but you may find downgrading from 78 is more trouble than it's worth.

I did try uninstall remove folder, install a older V of TB but same problem,,, wtf is this,,,

I installed TB 45 and this problem went away. I got a familiar window with an available "Confirm security exception" button. Updated to TB 78, it still works.

Sorry but this is over what i know and how to fix it,,, thx for the help but i have to find a new program that runs ok. Big thx for your help and goodbye TB sad to see you go but i cant have a email program that dont work...