Gmail has a system of reputation as you suggest. It is very likely that enough people marked their emails as spam, which the OOP could figure out on the postmaster dashboard if they were so inclined: https://postmaster.google.com/managedomains

It also goes the other way, if enough people click "not spam" and interact with your mails, your reputation gets better. I'm currently trying to do that with my personal email/domain - will probably take some time though. For now, my friends say my mails land in spam even though I get a 10/10 score on mail-tester.com and similar sites.

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aidenn0 2 hours ago | parent [-]

When I ran my own mail server, I was lucky to even make it to the gmail spam folder. More often it didn't even make it that far. From what I can tell, O365 is even worse though.

	▲	sva_ 41 minutes ago \| parent [-]
		They should go through, at least to spam - but your setup needs to be flawless, meaning you need to correctly set up the 'holy trinity' SPF, DKIM, and DMARC. Sites like mail-tester.com, learndmarc.com, or sending a mail to ping@tools.mxtoolbox.com (which will reply a report to you) are pretty useful for that. But yeah I have only limited experience I suppose. Having some mail correspondence with friends in the hopes of improving my domain's reputation to those mail servers. Oh and btw, I relay through my cloud providers mail delivery system - doing it from your own IP is probably a whole different league.