Maximum Message Size and Attachment Size

Hi, currently we are using Exchange 2010 and set the maximum message size to 10MB. When I sent an email with attachment of 10MB then it bounced back as message below:

This message wasn't delivered to anyone because it's too large. The limit is 9MB. This message is 10MB.

I am wondering how come the limit is 9MB instead of 10MB in the bounced back message? Is they anyway to set the size limit for message and attachment separately?

Sean