Recently I upgraded to my new, freedom-loving, pro-privacy Internet Service Provider Freedom Internet. I was already running a mailserver before I switched: postfix. For years I had no issues sending out e-mails towards other mailservers. After I upgraded though, it seemed I was unable to send e-mail to Microsoft e-mail addresses. I noticed it only after a few months, when I tried to reach someone with an @live.nl e-mail address.
So I decided to change my reverse DNS record from
something.domain.tld And behold, my e-mail was now being accepted again by Microsoft.
So although Microsoft's policies and guidelines state "Email servers must have valid reverse DNS records.", that is not the entirely true. I assume that Microsoft rejects e-mail servers with rDNS records with more than 2 dots. Beware if you use multiple DNS zones.