Well, since you are controlling size of attachment (not the size of message), why don't you use transport rules with below condition, action and exception because all messages pass through Hub transport server and process through transport rules.
Condition
When the size of any attachment is greater than or equal to limit
Action
Send bounce message to sender with enhanced status code
Exception
Except when the message is sent to users outside the organization
References:
Understanding How Transport Rules Are Applied in an Exchange 2007 Organization
http://technet.microsoft.com/en-us/library/aa996585.aspxExchange 2007 - Transport Rules & Disclaimer
http://exchangeshare.wordpress.com/2008/05/17/exchange-2007-transport-rules-disclaimer/Amit Tank | MVP - Exchange | MCITP:EMA MCSA:M |
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