[citrin_ru]

> It seems that these companies - Proofpoint, Microsoft, Google - make arbitrary and undisclosed rules for email delivery

I don't think that secret rules are the main problem. Two group of rules are public: don't send spam/phishing/malware and follow all relevant RFC (SMTP/MIME/SPF/DKIM/DMARC). The list of small things which affect mail delivery is long so I recommend to use tools like https://www.mail-tester.com/ which can highlight problems in your mail setup (a couple of checks are unnecessary strict though and some are relevant only to newsletters/maillists and not relevant for individual messages sent by humans but most items in their list you need to follow).

Everything else usually secret on purpose - so it would be harder for spammers to evade spam filters.

A much bigger problem is that any spam filter can mark a non-spam message as spam (False Positive) or spam message as ham (False Negative) and there is an inherent trade-off between FP and FN rates. It's easy to reduce one of them while letting another to increase. If I would build an anti-spam system I would target near zero FP and then will try to reduce FN to the extent it is possible not increasing FP. But looks like leadership of mail companies (like PP, O365 e.t.c.) targets low FN and cares much less about FP. Don't know why - may be their customers demand low FN and don't understand that asking for low FN they are getting high FP.

Having said that it is likely that your problem can be solved in 15 minutes (e. g. by removing you domain from a blocklist in which it can end up because of a system error - see above about FPs) if you can get a PP employee to check their logs. But that's the main problem - these companies don't invest nearly enough in processing feedback from non-customers, we are lucky if they hire a couple of (underpaid) contractors to process all non-customer feedback they are getting. So looks like the only way to solve the problem is to find some PP employee via friends of friends or find a PP customer who can file a support ticket.