I hope someone can help me to lock down my Exchange 2013 server. At the moment, it is being hit with a lot spam and I need to get it stopped quickly. I need the server to be able to send out to any domain (ideally. I can potentially set it to a large list and add to it when needed) but I need it to only accept email from a few specific domains. Right now it's accepting email from almost any domain. How can I lock this down?