Plenty of advice here, which I'm certain is well intended, but it feels like we're victim blaming here.

Things like "you cannot mix marketing and transactional email" are good advice, but they do nothing if you're using different subdomains but the same infrastructure (IP address) to send them out.

Microsoft is simply trying to squeeze smaller operators out of the market.

I run an open source project, we send out transaction emails from ome subdomain, and a newsletter to 50K+ subscribers once every 3 months.

I can't afford to pay for different/dedicated IO addresses for each. We use Scaleway for email delivery and it's constant trouble with two providers: qq.com who don't give a damn about non-chinese senders, and Microsoft, who are simply trying to break the ipen internet.

Before, we were on AWS SES. Guess what, we didn't even bother using different subdomains. We also didn't have dedicated IPs. Yet Microsoft did not block us because the sender IP was AWS.

It's pay to play, as simple as that.

Fortunately, we're an open source project, not a business. So when people reach out, I simply explain to them that they have chosen a mail provider that is openly hostile to small volunteer-run projects like us and that that choice has consequences. No emails for you.

IMHO we need to be more vocal about what's really going on here (the rent-seeking on the open internet by big tech), and less victim blaming (what about dns-sec).