I recently set up a quasi-email server in OpenBSD current with help from jggimi
https://daemonforums.org/showthread.php?t=7776.
The OpenBSD group is developing an alternative, OpenSMTPD, and the developer even dropped by the thread. IMHO it is more concise while offering a great deal of functionality. I would not be surprised if it becomes the default mail server for OpenBSD.
The one problem is the configuration syntax has recently changed so if you look at other
/etc/mail/smtpd.conf, there are a number of them out there, you will need to rely on the man page for your release.