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

Thankyou for your response but I have already tried this to reach to this error message. Earlier it was showing the tls security error message which I could bypass using these settings

I'm having the same issue. My email provider has updated their certificates and the wildcard cert they present does cover my home domain. (even though the DNS records are correct.)

For IMAP this isn't a problem the "security exception" window pops up and I can create an exception mapping my mail.<domainname>:993 host to the presented wildcard cert.

The SMTP side only presents the error and does not pop the "security exception" window that is expected.

I'm running TB 78.3.2

After updating Mozilla - YOU RUINED IT When I try to send with any security level I get this idiot cryptic message: Sending of message failed Peer's certificate has an invalid signature The configuration related to ..... must be corrected Not only is there no way to fix this, IT HAS STOPPED ALL INCOMING MAIL It doesn't even work reliably to set sending to normal, insecure password

The computer geeks at Thunderbird are stuck up snobs and don't consider their customers are not computer nerds like they are and shouldn't be messing in the config.editor when they don't know what they are doing. If you don't know what you are doing, you can mess something up and cause even more problems. They arbitrarily change how the program works without taking into consideration as to how it affects the users. If they do something like this they should provide an easy way for non nerds to correct it or override it without having to change things they don't understand. Increased security is worthless if the main people being locked out are the people who have to use it. Also they apparently knew this would cause problems and didn't bother to tell the users about it and provide an easy fix.

I get this type of problem maybe twice a year, I think my 15 year relationship with thunderbird is coming to an end... My business hasn't been able to send email for two days now and it sends just fine from my iphone.

Followed the config editor directions and am still getting the same error which I didn't get a few days ago with the same settings.

This happens when they "upgrade". Then they expect the users to be programmers and fix the problems. Hey, we are not computer geeks. We are not programmers. This is "sold" as a product for ordinary people not computer nerds. Expecting people with no programming knowledge to correct things is ridiculous. You shouldn't even install something that you KNOW will make it unusable to a lot of your users. Another thing, you need to improve you "automatic" detection for email. It almost never works.