There are plenty of spam filters available for OpenBSD and other BSD systems such as bogofilter or spambayes, but these tend to be overkill for a single user system like mine. Another possibility is bmf, which is described as aiming to be faster, smaller and more versatile than similar Bayesian filters. However, it doesn’t seem to have been updated since 2002 and I didn’t like it too much when I tried it out..
I prefer Quick Spam Filter (QSF), which I have used for a good many years – in fact, since before I changed from Linux to OpenBSD four years ago. Its last update was in 2015. I find that after training it is reliable, with only occasional false positives or negatives. It hasn’t been ported to OpenBSD but it compiles easily. It is available from its author.
The qsf documentation describes how to integrate it with procmail. I have a separate post with details of doing this with fdm – see the OpenBSD category or use the Search facility.