Your are still allowed. I self-host personal mail in a VM and still can send mail to gmail. The problem is that big players (Gmail/MS) care about an anti-spam

Your are still allowed. I self-host personal mail in a VM and still can send mail to gmail. The problem is that big players (Gmail/MS) care about an anti-spam false positives only when they affects a relatively large fraction of all messages. Small mail servers account for a small fraction of all messages, so big players don't care about them at all. I will not be surprised if they have SLI/KPI which allows them reject by a mistake millions of non-spam messages daily because it is a small percent of all messages they process.