DMARC failed according to Microsoft, have you tried the above tool yet?
Look at the Outlook Headers - or post them, they normally have a code in their that tells you the reason why it’s being flagged as Spam. Use this Mail tester it’s the best one I found so far.
The codes your looking for is BCL, PCL, SCL and SFV - normally attached to the x-forefront-antispam-report header - https://technet.microsoft.com/en-us/library/dn205071(v=exchg.150).aspx
Wow, I have to disagree about the unlocktheinbox service… That’s just one giant upsell. I get:
“Criticals: 1” (in big deep red)
But apparently I need to upgrade to Pro to see what it is?
Ha. Right.