I was hosting my own mail server, did not have open relays and I know 100% sure nobody on my server sent spam. It was fully configured with all the DMARC and SPF trimmings.
Yet one of my users needed to email users at live.com/outlook.com/hotmail.com and kept getting banned. Every time I was able to unblock it using an automated link.
One time it didn't work and I actually got through to someone. He was like "Yeah, your server doesn't send enough legitimate emails so it doesn't build up 'reputation'". This sounds ridiculous, not sending spam is not enough, you have to send a certain amount of legit mails to stay unblocked??
Anyway it kept happening so I eventually gave up :( It only happened with consumer MS-hosted emails addresses though. I had no issue reaching companies using M365 for business.
But email is just so incredibly broken... All the patches to kinda try and fix it are a mess. We need a whole new protocol.