Since release 6.4.0 OpenSmtpd has support for external filters.
It takes some time to understand its concept.
Here is a small showcase about it.

1. Install OpenSMTPD
2. Create a simple smtpd.conf

filter afilter proc-exec "/usr/bin/python3 /tmp/afilter/afilter.py"
listen on eth0 port smtp filter { afilter } 
action "inbound" mbox
action "outbound" relay filter {afilter}
match from any for local action "inbound"
match from local for any action "outbound"

3. Copy afilter.py and afilter_utils.py to the right location.
4. Start smtpd: smtpd -f smtpd.conf -d
5. Test your SMTP server: swaks -f [email protected] -t [email protected] -s localhost -ehlo home.local
6. Watch your output:

• Request: OpenSmtpd -> Filter color is purple
• Response: OpenSmtpd <- Filter color is green 6See results complete log in /tmp/opensmtpd_afilter.log

See also: ./sample_log.txt

1. Implement abstract methods like FilterSmtpIn->mail_from(ctx, address)

class ReportSmtpIn(IReportSmtpIn):
    @staticmethod
    def link_connect(ctx, rdns, fcrdns, src, dest):
        # your implementation
        pass

2. Listen only for special reports / filters

hooks = [
    ReportSmtpIn.link_connect, ...
]

• https://man.openbsd.org/smtpd-filters.7
• https://poolp.org/posts/2018-12-19/more-on-opensmtpd-filters/
• https://poolp.org/posts/2019-12-24/december-2019-opensmtpd-and-filters-work-articles-and-goodies/