He’s been using the same IP address for his mail server for 15 years. It’s possible his IP is on a subnet range that regularly gets blacklisted by Gmail due to other actors on the subnet sending malicious email/spam to Gmail.
Presumably, Google's smart enough to identify individual IPs in the range that don't send spam. I have a real shitty neigborhood IP wise, having my mail server on a $1 VPS. Public Outlook.com drops all my mail regardless of content for example, citing spammy IP range. Yet gmail somehow learned after a while to pass most of my messages through.
The real problem is lack of provided reasons for blocking, so people waste a lot of time trying to figure it out by guessing and trying random shit.
