Our emails are blocked by Hotmail and Outlook and Microsoft refuses to tell why ("I do apologize, but I am unable to provide any details about this situation since we do not have the liberty to discuss the nature of the block.") and asked us to check this link -> https://sendersupport.olc.protection.outlook.com/pm/troubleshooting.aspx
We've set up our SPF, DKIM and even DMARC (Gmail and mxtoolbox.com confirmed all as pass).
But under the link above there's a section below.
Confirm that your DNS is set up correctly Try connecting to mail.hotmail.com via port 25. If you are unable to connect, then attempt to telnet over port 25 directly to our email servers (MTAs). You can find the current list of our MTAs by querying "nslookup –q=mx hotmail.com" from a command prompt (this should work in a variety of Operating Systems). Currently, the addresses for these servers are mx1.hotmail.com, mx2.hotmail.com, mx3.hotmail.com and mx4.hotmail.com. If that doesn't work, try connecting directly to the IPs. If you are able to connect directly to the IP and not mail.hotmail.com, then it is likely that there is an issue with your DNS server.
I tried the above and my server can't connect to mail.hotmail.com via telnet 25. Is it necessary to confirm the DNS via telnet 25? I also nslookup of hotmail MX failed.
[root@server ~]# nslookup –q=mx hotmail.com ;; connection timed out; no servers could be reached
And the URL above also asked us to check our sender score at https://www.senderscore.org/get-your-score/ and our IP and domain name was clear (all green checkmarks).