I've been experiencing this issue since the initial release of iOS 18, and it persists through subsequent updates. The most recent version I tested was iOS 18.2 beta 4. The result is always the same: using self-signed certificates does not work (-9 808) with the default Mail app. On the server side, I’m using Linux with Sendmail and Dovecot. Is there any official statement from Apple regarding this issue? A response would be greatly appreciated, considering how long this problem has been ongoing.

Previously, self-signed certificates worked seamlessly with the iOS stock Mail app. However, since version 18, something appears to have changed, causing them to stop working, with error code (-9 808). After wasting a significant amount of time troubleshooting, I managed to resolve the issue, largely thanks to the previous recommendation by . The solution involved regenerating the server certificate with SAN (Subject Alternative Name) records. Then, installing the updated certificate on the phone and removing and re-adding the mail account seemed to do the trick, at least in my case. **_ If Apple has indeed changed the requirements for self-signed certificates, this should be explicitly communicated to users, in my opinion._