Re: This has been going on for years!
Same, I run and small web/email host for some small business's in a small town in northern ontario. Running since 2006, no issues with email. All setup with DKIM, SPF, DMARC, the works. Using an open source spam solution (mailcleaner) for years, prior to that Barracuda Email Security.. Never had an issue.
One day, users report issue with emails, same issue as the article. Event offered to have the agents remotely connect to show the issue, they remoted in, and proceeded to uninstall my antivirus saying it was causing the issue. After months of back and forth, frustrated customer, I was forced to change external Ip's for clients, and the host in general. Still working on IP rep now, daily battle to get back to the stability I had before. I had logs, proof of an MS Issue, but they refused to look at it or work with me.
I'm going to use the "Secret List" that was mentioned, archiving this article as ammo for my next battle with MS.
Biting the hand that feeds IT
