Helo again, I have worked on a package called the OpenSMTPD-framework for developing Golang filters, it is an unofficial project that's only supported by me, don't bug OpenBSD with bugs :-)
It is available at: https://github.com/poolpOrg/OpenSMTPD-framework It can currently be used to develop table backends and filters. It contains an example table and an example filter but I have already implemented a filter-kicker which took me about thirty minutes to get running and which you can find here: https://github.com/poolpOrg/filter-kicker I will likely convert the popular filter-rspamd and filter-senderscore to this framework soon, so it is easier for me to maintain them all. Feel free to test, contribute and let me know if you use it as I'm still at a point where I might break things to ease the API. Cheers,