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The sending IP remains very relevant; it may be in a third-party blacklist (RBL) or site local blacklist due to prior spam from said IP or even nearby IP(s). Let's have a look through /var/log/maillog... okay that didn't take long.

    $ rubbled 86.54.42.238
    86.54.42.238 zen.spamhaus.org. XBL (exploit)
    86.54.42.238 zen.spamhaus.org. 127.0.0.9
    86.54.42.238 zen.spamhaus.org. SBL (spammers)

Spammers can setup DMARC, and have too many domains, so blocking by IP or ASN remains relevant (no legit email from that spammy country? Ban the country!). Reverse DNS is also important, as spammers have sent too much spam (shocking, I know) that some users complain about, a lot, so: no valid reverse DNS, no service. IP addresses or domains that are "too new" may also be a problem, or some sites will want you to fill out random webforms or talk to their support idiots (Hi, Microsoft! No, me logging into some cloud thing of yours was utterly irrelevant to the problem), and all this and more amounts to a lot of rakes you need to not step on to get email setup right.

Yes, I self-host email. Gmail was routing OpenBSD mailing list traffic to the spam "folder", and self-hosting that email was easier than fighting with some rink-a-dink web UI.

Oh, one time about half the customers were in Google and the other half in Microsoft and Google and Microsoft were having some mail snit so yeah good luck getting some of those mails through. That took a while to clear up, and what can you do?