It stuff: Having problems sending email to hotmail.com, live.com and other Microsoft free email domains? Read this!

Do you run your own mail server, but any email you send to hotmail.com, outlook.com or other Microsoft hosted domains is rejected outright, or in rare cases sent straight to the 'Junk' folder?

You typically get a reject message like this:

Action: failed
Status: 5.7.1
Remote-MTA: dns; eur.olc.protection.outlook.com
Diagnostic-Code: smtp; 550 5.7.1 Unfortunately, messages from [your.ip.add.ress]
    weren't sent. Please contact your Internet service provider since part of
    their network is on our block list (S3150). You can also refer your
    provider to http://mail.live.com/mail/troubleshooting.aspx#errors.
    [Name=Protocol Filter Agent][AGT=PFA][MxId=11BD1ECE2D0673DE]
    [DB5PEPF00014B9C.eurprd02.prod.outlook.com 2026-04-13T15:26:24.418Z
    08DE8F6725EF2E9C]

You will note the keywords: Please contact your Internet service provider since part of their network is on our block list (S3150)

My mail server is configured exactly as per Microsoft's guidelines in their various support documentation.
Back in June 2021, they responded to my support request, stating that 'mitigation' had been implemented for my mail server IP address.
I was then able to send email to these domains for a short time, however fast forward a few weeks and my email was once again rejected and my IP address was back on their 'block list'.
My IP address is a proper static IP on a hosted server (I have a /29 block assigned by my ISP).
It is not listed in any blacklist, at least for the past five+ years that it's been assigned to me - and I'm not aware of it being listed previously.
I don't send marketing emails (or any other spam) - the server is used purely for private and small business communications (I run our apartment management company).
I have SPF records, sign my emails with DKIM, use MTA-STS, and all DNS records are 100% correct.
Every request to Microsoft for support results in the same rhetoric - I am referred to their "enhance_deliver.pdf" document (which has several errors and typos).
As recommended by their support team, I signed up to their SNDS "Smart Network Data Services" service to check for spam reports related to my IP address (although I don't send enough email to these domains, those that I do send email to are regular contacts).
There was a moment of considerable irony, when my email system tagged their SNDS confirmation email as spam. Why? Well the 'HELO' name didn't match the IP....
So seemingly at the end of the day, with Microsoft it's a case of "don't do as we do, but do as we say - and if you're lucky, we won't send your email to Junk".
The Microsoft support team constantly recommended that I (or my ISP) sign up to (and pay handsomely for!) a Validity subscription.
Why would I want to do that when my systems are configured 100% anyway?
Do Microsoft have a vested interest in Validity?
I have no problem sending email to any other provider. Gmail, AOL, Yahoo etc. all work just fine. It's only Microsoft free email accounts - I have no issue sending to companies using Microsoft 365.
It's also worth noting that their free services are using IPv4 only - Microsoft 365 uses IPv6 and IPv4 - I guess it's a bit more onerous to block IPv6...