Mime encoding for exchange 2007

I am using Symantec cloud to block email attachements over 10 meg. Likewise I have a ten meg restriction on my exchange 2007 server for inbound mail. When I send a 10.2 meg attachment symantec does its job and replies back to sender and recipient with a notification the mail was blocked.

When I send a 9.7 meg document symantec allows it through however exchange blocks the mail and rejects it with an error to the sender. I had to lower the attachment size to 7.1 before I could get it through exchange even though exchange is set to 10 meg.

It would seem like exchange is calculating the whole mail size and adding MIME encoding overhead of 33% or thereabouts.

I was thinking about increasing exchange limit to roughly 13/14 meg to account for this but I am unsure if 1) MIME encoding is the same calculation for all attachment formats. 2) is there anyway to tell exchange to look at only actual attacment size of the attachment.