Managed to generate a cert with my full server name, and now Mail works on the beta. So it appears that any certificate that must be "trusted" will fail. Updated the ticket with this info.

It's not just self-signed certs. I switched to a "Let's Encrypt" cert which only has my domain name in it (and not the name of the mail server). So Mac and iOS still need to "trust" the cert because of the name mismatch. This works on Macs and on iOS 17 devices, but not on iOS 18. So I think it's any cert that needs to be "trusted." I updated my ticket with this info.

Feedback generated: FB15168540. Referenced the earlier FB13825638, but I can't see that report myself).

Just tried the 18.1 beta 4, acts the same, so not fixed yet.

I have the same problem. iOS devices are screwed, but I can "trust" away on my Macs. I tried the workaround but I'm not good enough with certs to get it to work apparently. For now I am using a 3rd-party email client, but it's not ideal. I would rather go back to Mail. Apple, please fix this. My symptoms are that I can "verify" the account after "trusting" the cert, and the initial download of my mailbox works fine. But after the initial load, all other connects fail with the "bad certificate" -9808.