Some ISPs, in an effort to reduce e-mail spam originating at their customer’s IP addresses, will not allow their customers to communicate directly with the recipient’s mail server via the default SMTP port number 25. In this case the customer has no choice but to use the smart host provided by the ISP. A growing number of systems also verifies the sending system against known lists of cable modem and DSL networks and will not accept SMTP connections from these systems to reduce the amount of incoming spam. Field test have shown this can have a sizable impact on the number of spam messages one receives and it is expected to become more and more common. Source http://en.wikipedia.org/wiki/Smart_host .