From what I can tell it's just that small domains are guilty until proven innocent. However it seems at lower volumes you can never send enough email to get past the thresholds they've set so you're forever guilty in their eyes. Very convenient.
I've also signed up to both Microsoft and Google's email tools and neither shows any information because I don't send enough mail while they still keep making my mail as spam. It's getting to the point where I'll probably have to use a 3rd party SMTP rely as the family don't appreciate their mail going to spam while all their friends on GMail go straight to the Inbox.
A nice new addition is Microsoft is flagging any links I include in my message as "suspicious" and giving the users scary warnings. Meanwhile I can see they still take the opportunity to scan and fetch the URL and yet still can't determine it's safe.