Hi! I've seen on proxy protocol spec ( https://www.haproxy.org/download/2.3/doc/proxy-protocol.txt) the section "Validation" with a list of tested implementations for some protocols.
Right now on SubEthaSMTP we have created a new release with support for proxy protocol (https://github.com/davidmoten/subethasmtp/releases/tag/5.2.7. Please note that this is a maintained fork of an unmaintained project that is a fork of an unmaintained project). SubEthaSMTP is a powerful yet simple open source SMTP server. How can our implementation be validated? Thank you Diego Salvi