Problem: One of our Exchange accounts were 
compromised and then used to send mass amounts of spam through our system. We shutdown the account and cleaned up the mail but wanted a way to limit the number or recipients so the spammer couldn’t send out so many at the same time.
Solution: The default for Exchange 2010 is 5000 recipients. Since we only have 200 mailboxes, we could lower this a bit to hopefully curtail these mass sends. To do this, go into Exchange Management Console and go into Organization Configuration. Then right mouse click on Hub Transport and select properties. Under the General tab, you will see the max number of recipients option. Choose a number that suites your organization and click Apply.