Security

That’s usually the mail server certificate, though there can be other causes.

The mail server involved would seem to have an error with their TLS security certificate, or there might be a commercial first-few-hops VPN service (unnecessarily) involved and commercial first-few-hops VPN services are unfortunately far too often doing something sketchy with network traffic. It’s also possible you have loaded a personal certificate for signing and encryption, and that needs to be updated.

If you have an add-in VPN client, remove it entirely, restart, and try again.

If you do not have an add-on VPN client app installed, check with the folks maintaining the mail server you are using. (It’s also possible to check the TLS certificate in use yourself to verify its validity, but that’s a little more involved.)

Adding and using personal certificates on iPhone and iPad is possible, but seemingly also fairly rarely done.

I’m a unsure what sort of misbehavior is being reported here.

Your certificate? Could you explain a little more about the situation? Certificates can be used in lots of ways.

Is the iPhone patched to current? (iOS 15.5 is current.)

If it’s a purchased certificate or a LetsEncrypt on a website you’re administering… Contact the certificate vendor, and determine if (for instance) you need to include an intermediate certificate within the certificate chain that the website returning with the purchased leaf cert.