The issue appears to be with Microsoft's spam heuristics algorithm. Often spam seems to bypass their filters. I assume either they have an "I'm too busy, just let it pass" rule, or there is something in specific emails that seems to break their algorithm.
I have downloaded all the spam emails from my paid hotmail.com inbox and uploaded them to several open-source IMAP filters, and they picked up and filtered 100% of the emails that the Microsoft filter passed into my inbox. The spam score in each case was REALLY HIGH.
Some examples passed into my Hotmail inbox:
- Emails purportedly to be from Microsoft but with <no sender address>, saying "your device is under attack, all your files will be deleted"
- Various Antivirus emails telling me my subscription has lapsed
I have a friend who works for Microsoft and he said it's a running joke inside the company about how poor their spam filter has become. It used to be pretty much watertight.
Come on MS, realise there is a problem, and FIX IT!!!