This is the hallmark of Content Filtering in the later versions of Exchange. Here's how to disable it:
• From the Exchange program group:
• Open the Exchange Management Console. Next, depending on the server type, select either or:
• Microsoft Exchange | Edge Transport | Anti-Spam
• Microsoft Exchange | Organization Configuration | Hub Transport | Anti-Spam
All of the filter types are enabled by default, and if the server's mail traffic is filtered by MailRoute, it is not necessary to keep them enabled.
• Open the Content Filtering anti-spam filtering type and go to the Action tab.
• Then uncheck the box next to "Reject messages that have SCL rating greater or equal to:"