The SNDS and JMRP from Microsoft has always been useless for me. You have to send out massive amounts of messages before SNDS or JMRP gets triggered and none of our servers send out that many messages. (I have no what the goldilocks number is for "massive amounts" but it's more than 100 per day or so).
And that's the issue I have with these programs. Our servers get blocked by Microsoft, but we get nothing from SNDS or JMRP - so what's the point of these programs?
Microsoft - and any of the other major players in the Email Service Provider industry - can block your server's IP for one reason: Because they can. The everyday human being recognizes the name Microsoft (or Google or Yahoo) and does not believe they can do anything wrong because they're a massive company. So if your server's IP is being blocked by Microsoft it's OBVIOUSLY because of a problem with your server, it can't possibly be because Microsoft is wrong.
The key with Microsoft is that you have to submit a ticket regarding your IP address being blocked. It doesn't really matter if your a SNDS or JMRP member, I gave up on that a long time ago. After you submit a ticket, in a few hours you'll get a reply saying something to the effect that they don't see any issues with the IP address. This is a canned response. Reply back to that ticket and ask them to escalate it. Then in a few more hours, you'll get a response that the block has been mitigated. You have to reply to that first message to get anything done.
For better results, I'd encourage you to sign up or use a free outlook.com email account solely for submitting this ticket and replying back. This way you're within Microsoft's walled garden. Can't be blocked for receiving their message or sending a reply to their message if you're using the same ecosystem they are using to generate and send those messages.
Outside all of that, if you're not already a member to the MailOps mailing list - you may want to consider that -
https://www.mailop.org - you can generally get connected to someone that actually has controlling power to these Email Service Providers from that list.