That being said the filesystem certainly is the weakest part of the OpenBSD and given the uptake in filesystem designs with ZFS, Btrfs and Bcachefs it is interesting to see that OpenBSD is left behind.
For a personal mail server, or even small business, it's not really an issue, you're likely not going to have terabytes of email.
Pre-2005 running FreeBSD was a nice little "secret" that allowed you to run a rock-solid OS without drama or headaches. However, professionally nowadays I've accepted the fact that linux "won" and I don't want to deal with the headaches of finding people that can admin something niche, on top of so much tech tooling being designed on and around linux. Most of my work is done supporting docker containers in some way, so why fight even if it's possible to run docker on FreeBSD...