Mail.app refuses to send e-mail

I have just upgraded to Monterey (12.0.1 (21A559)), and cannot send e-mails. Mail.app tells me that the certificate is not valid.

We own our own domain and host the primary MX here, and this worked on my previous Mac.

It is actually a real certificate obtained from a bona fide registrar, and it works fine in our web server. But I guess it has to be modified in some way to be a mail server certificate.

Alternatively, can I accept the certificate permanently? If so, how as the option to do is currently greyed out.

Incidentally, it's the same on my iPhone and iPad - can receive mails but cannot send them.

Thank you.

I fixed it. I hadn't looked closely enough, and the cert expired in 2018. Grabbed the certs from my web server, scp'em them to the mx, and configured sendmail.

Fixed!