What's a Smarthost?

A Smarthost is essentially a system to which all messages are forwarded, regardless of their ultimate destination. Examples of Smarthosts are AntiVirus appliances, Content filters, Sender/Recipient filters, and Spam filters. In addition, some ISPs and some of their service levels require customers to send all outbound mail through their systems instead of directly to recipients. A Smarthost is responsible for receiving mail and forwarding to the appropriate system. Note that it too may send messages to a Smarthost or directly to a recipient system.