Still not working. I've dumped the profile and reloaded it, and even generated a new CA cert and resigned mail certs. Mail will not trust or ask to trust after the profile is added and trusted. Worked fine prior to IOS 18.

Same problem, but I may have a workaround. My old certificate used wildcard SANs. I have just updated with a new certificate using NO wildcards. I then added a new account, and it allowed me to trust the new certificate. I have to test to see if this is actually working for that account, now, and if it fixed the other accounts that use the same certificate. But at least it's progress.