Now that Microsoft has merged their Office365 and Hotmail/Outlook.com platforms, this should apply to anybody sending to either platform. Microsoft calculates a "BCL" (Bulk Complaint Level) for a sender's IP address or sending domain name. (Which? I'm actually not sure at the moment. Let's assume both for now.)
The BCL score is a 0-9 score, where higher basically means "sent by a bulk sender, and more spammy." See this Microsoft Technet article for more details.
How do I tell what my BCL score is? Select "View Message Source" on an email message received at Microsoft Hotmail/Outlook.com. Find the "X-Microsoft-Antispam" header. Here's an example:
That first entry -- BCL:0 tells us that this message is from a sender that has a BCL score of zero. (This message is not from a bulk sender.)
What do those other entries mean? PCL means "Phishing Confidence Level" per this document. So it's good to see that is zero. The rest? I'm not sure. I'll share more as I learn more.