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Usually your IP.

I've been running my own mailserver since 1999.

I've had SPF for a while but DKIM and DMARC only since last year. Had no issues sending to the big players.

You can always switch to a smarthost if you run into these problems. No reason to give up on self-hosting.

It's so random who this hits. I've had very little problems. YMMW. If you really want to run your own mail server and can stand to try, just try it. It may work with your IP. (Or whatever heuristic which the main providers go by.)

In my case, sending to Gmail works via IPv4 but they reject anything I send via IPv6. That was fun to figure out.