Bulk mail is often mistaken for spam and is starting to become a larger problem for organizations. EOP is not very aggressive out of the box when it comes to bulk mail because this type of mail falls into a grey area. Some organizations will want to receive this type of mail, whereas others will not.
Over the last few months we have greatly increased EOPs ability to detect bulk mail which you can take advantage of starting today. This new system is based on a scale which gives customers the ability to set the aggressiveness of bulk mail detection to meet their specific needs.
X-Microsoft-Antispam is a new header that is stamped on all messages traversing Exchange Online and only started appearing in messages few months ago. This new header currently contains two published values to help better detect bulk and phishing emails.
- BCL – Bulk Complaint Level
- PCL – Phishing Confidence Level
Read more at source: http://blogs.technet.com/b/exchange/archive/2014/09/25/take-advantage-of-eops-new-bulk-mail-detection.aspx