Setting Outbound Attachment Limit in Entourage 2008

I have tried using the built-in outgoing rules in Entourage to limit users trying to send email (POP Client) attachments larger than 10MB. I can not get it to apply the rule before it tries to send it so it is pointless. The msg is sent out regardless even if I have a rule that deletes the msg. It does get deleted but only after. Anyone know how to create a script that would force Entourage to check the rules and have it prevent the msg even trying to go out if the criteria is met?