[mewmew07]

I think some of the entries in the Inbound section will inform how to deal with spam, I saw some references to spam assassin there.

https://stalw.art/docs/category/inbound

[tankenmate]

In particular the DATA (or BDAT) stage configuration; it runs a command and send the headers / data to stdin and receives back the modified message from stdout.

https://stalw.art/docs/smtp/inbound/data/#content-filters