Mail SSL Cert problem

Had a client that was using mail.domain.com without a problem with Thunderbird.

I did a kernel upgrade tonight along with upgrading PHP to 7.3.21, and he called saying he was getting a certificate error.
Saying the certificate didn't match the main hostname.
He didn't make any changes.
And I can't see how a kernel & PHP update could have caused this.
But I did do a server reboot after the kernel update.

Had him delete and recreate mailbox, same error.
Same error before mail. became an option for the SSL certificates.

Anyone have any ideas what might have happened?

Hi, just try to reinstall the SSL via Server Settings -> Change hostname
I guess something went wrong and the certificate was deleted or replaced with the self-signed one.

The problem isn't the server hostname certificate.

Client was able to connect using SSL to his domain name. e.g. mail.domain.com, not the server name.

Now it;s asking for the server name for IMAP access, but is OK for SMTP.
Which is weird, and seems backwards.

Try to reissue certificate for mail.domain.tld, maybe it will be fixed.

Thanks, CoriaWeb Hosting help fix it when we were talking about the SpamAssassin problem also.