Custom transport agent to redirect emails over certain size.

Is there a custom transport agent that can do the below?

This is for SBS 2008.

Hi

I have created a transport rule that redirects emails that contain attachments greater that 11mb, however this only works when a single attachment is 11mb or over. I'm looking for a rule that takes into account the overall size of the email... e.g.

email may contain 5 photos all at 5mb, so the overall size of the email is 25mb. this email needs to be redirected to support where we can resize and on-forward to intended recipient.

Please advise