[IMGate] help being abused!

[Omar K.]

It appears that some server is abusing my  Sender Address Verification.
 
They are sending thousands of emails that appear to be from:hotmail to
non-existent email accounts on my server.  My IMGATE then goes to check if
that email does in fact exist in hotmail causing a dictionary/harvest
attack on hotmail.  
 
For now, I have blacklisted the IP address.  But how do I prevent this from
happening again as im sure the abusive IP address will change?