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.