The same problem encountered with iOS 17 beta 1 and beta 2 is back:
Unable to create a secure connection to the server ("bad certificate format" -9,808).
FB13825638 (iOS 18 REGRESSION: Mail no longer accepts a self-signed certificate from my mail server (AGAIN))
Problem remains in iOS 18.0 beta 2 (22A5297f). No change.
Still occurs on iOS 18.0 beta 3 (22A5307f).
Problem remains in iOS 18 beta 4 (22A5316j). No change.
Problem remains on iOS 18 beta 4 v2 (22A5316k). No change.
Just hit this as well...
I got the same with 22A5326f ... I even reinstalled the profiles and hit the "trust" button but nothing solves it.
As you said, behavior is still regressed in iOS 18.0 beta 5 (22A5326f)! Added more comments in the thread linked by DTS above about the similar problem that occurred in iOS 17.0 betas.
Still broken in iOS 18.0 beta 6 (22A5338b)!
Haven't been able to use mail on my iPhone since WWDC :(
Sadly, this regression has shipped in iOS 18.0 (22A3354). I am not optimistic that we will see a fix going forward. :(
I just experienced this problem after updating to iOS 18 - can't access my email - "bad certificate format". :-/
js.
Our certificate isn’t self-signed but we use TLS 1.0. Ihave same trouble with iOS18. 18.0.1 (22A3370) also with it.
On 8 October client reported that latest iPhone iOS 18 Beta has fixed the problem but then same day reported that it had broken again! Hopefully trying iOS 18.1 RC later today, fingers crossed.