The Outgoing Server is the platform that allows clients to send outgoing e-mail messages to any valid e-mail address. The server performs two basic but important functions. First, it verifies that anyone attempting to send outgoing e-mail through the server has the right to do so. Secondly, it sends the outgoing mail and if undeliverable, sends the message back to the sender.

This is why you need to specify both the outgoing server and the incoming server when you configure your e-mail.