Installieren mit:

apt install opensmtpd opensmtpd-filter-senderscore

Dann die Datei /etc/smtpd.conf anpassen. Meine sieht jetzt so aus:

# This is the smtpd server system-wide configuration file.
# See smtpd.conf(5) for more information.

# Zertifikate:
pki mail.example.com cert "/etc/letsencrypt/live/mail.example.com/cert.pem"
pki mail.example.com key "/etc/letsencrypt/live/mail.example.com/privkey.pem"

filter check_dyndns phase connect match rdns regex { '.*\.dyn\..*', '.*\.dsl\..*
' } \
    disconnect "550 no residential connections"

filter check_rdns phase connect match !rdns \
    disconnect "550 no rDNS is so 80s"

filter check_fcrdns phase connect match !fcrdns \
    disconnect "550 no FCrDNS is so 80s"

filter senderscore \
    proc-exec "filter-senderscore -blockBelow 10 -junkBelow 70 -slowFactor 5000"

filter rspamd proc-exec "filter-rspamd"

table aliases file:/etc/mail/aliases

listen on all tls pki mail.example.com \
    filter { check_dyndns, check_rdns, check_fcrdns, senderscore, rspamd }

listen on all port submission tls-require pki mail.example.com auth filter rspamd

action "local" maildir junk alias <aliases>
action "outbound" relay helo mail.example.com

match from any for domain "example.com" action "local"
match for local action "local"

match from any auth for any action "outbound"
match for any action "outbound"