I've noticed that we're receiving a new header in our e-mail coming through EOP with the text "X-Microsoft-Antispam." This appears to have started on May 31, 2014. We also saw a drop in Spam e-mail from ~180,000 messages to 9,000 messages between May 30 and May 31.
Were there changes to the EOP engine during that time, and how do we interpret the X-Microsoft-Antispam header values?