I think the major problem here is IP reputation. Especially if you are hosting on public VPS or worse your home network. The root of the problem is spam abuse.

I think the major problem here is IP reputation. Especially if you are hosting on public VPS or worse your home network. The root of the problem is spam abuse. Spammers have no problem to spin up random VPS to send spam. They can set rDNS, SPF, DMARC too. There is enormous amount of spam. Of course it should be handled on message-by-message basis, using proper antispam and ML. But it looks like some providers simply block various IP prefixes. :( It makes their work easier and it is marketing to use their services "which work".