Eliminate\reduce Spamming from my domain.

Hi Someone is sending emails from our domain (not from our systems but our domain) and we get all the bounce back. I want someone with experience to do the next things: 1) configure DOMAIN DKIM and other related configurations - so recieving server will be able to verify if source is approved by our domian. 2) change our incoming and outgoing email service to a different one - free and reliable (open for suggestions), we are currently connected to mandrill. Plz only experienced in the requirements. TNX