Sorry, my implied question was : is what I observe and assume correct and there is no way now with iOS 18 to have the mail app use an IP directly, or is there something special/extra I should add in the cert? (the SAN field of the cert contains the IP). I agree it's not really a dev question (I'm an embedded dev myself) but I'm afraid it is a too technical question for support forum, hence I tried here as maybe somebody has experience with related certs/security management changes in iOS18 and could give me a hint.

I’ve asked as well on the app support forum. As a dev, I often answer questions on my github that are config or usage related. And yes, these are my apps so it’s different but these questions are normally asked on dedicated support forums not on GH. Anyway, I’m not sure we need to make a big fuzz about that, let’s say then that my ask to devs in general here, related to my issue, is ”please don’t do the same in your apps and make sure that when you use certs, you don’t forbid use of IP-addresses directly”.