Contrary to the complaints listed in that link above, I've run my own email servers also for decades, and delivery is actually MORE RELIABLE than going through "big name email provider".
He's probably consistently making some simple configuration error or someone has broken into his servers, and its being used for mass spamming, and thus being blacklisted.
It is critically important to maintain the security of your server when doing this. Use ipsets/iptables to mass block parts of the world that you don't want connecting to you, fail2ban on ALL authenticated services, and make sure that there's no open relay!