Prevent Spam

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Hi,
i have a question.
we have had lot of problem because of spyware attack on the user pc.
It has capture user mail password and start sending tousand of mail around the world.
So now we are finished in some blacklist and our mail are rejected....
is there any way to prevent this?( i mean...the pc il proxy and antivirus protected so i hope is possible to do something on a server level)
Maybe the right way is the possibility to automatic sospend an account if start sending too much mail in few second...
tanks and sorry for my english