This is a simple implementation based off my opensmtpd-filters-go library.
Debian ships with greylistd, a simple greylist management server written in Python. greylistd implements a simple protocol over a local socket that manages the greylisting of IP addresses, HELO hostnames and RCPT addresses. This filter currently only uses the sender's host's IP address.
Once the packages are uploaded, you will be able to install on Debian Buster like this:
echo "deb http://repo.maurus.net/buster/opensmtpd/ mn-opensmtpd main" > /etc/apt/sources.list.d/opensmtpd-greylistd.list
apt-get update
apt-get install greylistd opensmtpd-filter-greylistdIn your OpenSMTPD configuration activate filter-greylistd:
filter "greylistd" proc-exec "/usr/lib/x86_64-linux-gnu/opensmtpd/filter-greylistd"
listen on "127.0.0.1" port 25 filter greylistd