re :''The first error message was about the certificates, which I allowed and then this was the resulting error message:'' What is the resulting error message ? Is the mail account set up as a POP or IMAP mail account? Please check the following. If you cannot send nor receive anything from Thunderbird and this occurred immediately after an update, it is possible your Firewall is not allowing Thunderbird access to the internet. The 'Firewall' may be part of your Anti-Virus product. What Anti-Virus product are you using? I have known Norton to get confused before, it thought the update was something entirely different and blocked Thunderbird. I had to exit Thunderbird first, access Norton AV, locate 'Firewall' where it said Thunderbird was allowed. So you would expect all to be fine, but Norton thought this was an earlier version for some weird reason. So I blocked Thunderbird, said ok. Exited Norton. Then accessed again and reset Thunderbird as 'allowed'. ok'd it and then restarted Thunderbird. Resetting Thunderbird as allowed now made Norton think the current version was ok and it all suddenly started working like magic. So it is worth checking this out with your Firewall.