As a security-focused user I chose iRedMail, especially because of the OpenBSD-support.
Using "gpgit" (https://gitlab.com/mikecardwell/gpgit) I managed to encrypt all incoming unencrypted Mail to have a even higher level of security for certain mail accounts to minimize the impact of severe security issues.
It would be even better, when things like encryption of incoming and local E-Mail and automatic encryption of outgoing (S/MIME, PGP) E-Mail would be possible in iRedMail. And maybe even some PGP-encrypting web interface/ form (like https://encrypt.to) for incoming mail by senders without PGP and (password-)encrypted web interface for recipients without available PGP-key.
I looked at the project "ciphermail" (https://www.ciphermail.com/gateway.html) whose feature list looked promising in this regard, but unfortunately it was using a quite unstable and unsecure looking Apache Tomcat/ Java Setup, which I am not willing to put my trust into.
Maybe the developers/ community are interested in adding such functions to iRedMail in the long run, too.