Moderate messages sent to multiple recipients

Hi,

Wondering if anyone knows of a way to moderate outgoing emails that have 10 or more recipients?  We have a business case where we want to ensure any bulk communications are approved by the corporate team before being sent out.  

There are moderation rules I can apply in the ECP, but I can't find any options for number of recipients. (this isn't groups - it's individual email addresses too - it's due to the way our CRM works).

Thanks 

Luke