Earlier quoted context omitted.
I appreciate the response. From someone who has operated a mail server since 2002, it comes across that Gmail does not care about cooperating with small but legitimate mail servers. SPF, DKIM, reverse DNS, no blacklists, no open relay, longtime ownership of IPs, etc etc. Using various mail testers returns a 10/10 deliverability score. And yet, messages sent to Gmail always go into the spam folder, or are never delive…
I also have a 15+ years old set of mail servers and I host mail domains for 10ish friends and family. In fact, I once wrote how to set this up ( http://flurdy.com/docs/postfix ) I no longer use Gmail myself but half of my users relay some aliases to their main Gmail account. No problems with that, except my servers continuously get rate limited by Google: Our system has detected an unusual rate of 421-4.7.0 unsolicit…
But it does feel like they are treating nearly all minor relays as spam relays. We can not block 100% of spam before we relay onwards to Gmail as that would mean too many false positives emails get blocked, but most try to block as much as possible.
In my case, my servers probably block 99% of spam, but some will get relayed, and most of those ends up being handled by Gmail's even better Bayes scoring and filtered to end users' spam folders.
I don't know what the automatic threshold limit is to be on their naughty list, but it must be very low, as in double digits per day.