Hi Daniel!
In my case, if I connect over imap things work, but over imaps the imap process dies
We're on Dovecot 2.3 and can't get IMAPS to work on new iPhones / recent iOS versions. If the mail account was set up on an old iOS version, it continues to work even on latest iOS versions. But we cannot onboard new users on new iOS devices.
We use IMAPS only. Unencrypted IMAP is entirely disabled due to EU GDPR. Maybe the problem is related? (IMAPS vs. Apple?)
Our certificate (chain) is fine. It works flawlessly on Thunderbird (Windows and Linux), MS Outlook (Windows) and K9Mail/Thunderbird (Android) without security warnings etc.
The problem exists only with modern iOS devices and the default iOS Mail App.
Our workaround is buying Android phones only.
Reg