This module is actually a set of rules that are historically implemented as a separate plugin. These rules are intended to filter some known bad patterns in hostnames, received headers, helo domains and reverse dns.
The module configuration is quite limited: you can disable or enable hfilter rules by switching the corresponding groups on and off as following:
# local.d/hfilter.conf helo_enabled = true; # helo patterns (e.g. unresolved domains, bare ip addresses etc) hostname_enabled = true; # hostname patterns url_enabled = true; # url rules (e.g. `HFILTER_URL_ONLY` or `HFILTER_URL_ONELINE`) from_enabled = true; # mail from rules rcpt_enabled = true; # recipient rules mid_enabled = false; # message id rules (e.g. `HFILTER_MID_NORESOLVE_MX` or `HFILTER_MID_NOT_FQDN`)