What is the difference between Mail Server Forwarding and User Account Forwarding with the IMAP / POP products?
Mail server forwarding is used to forward all mail from one mail server to another mail server for a paticular domain. The destination mail server (an MTA, mail transfer agent) must have a the abilities to accept mail for the domain. The mail is not actually delivered to the mail forwarding server, it is just passed through or routed through the server. This service is extremely popular for people that want to accept mail on a port other than 25 if their IP is blocking port 25. This solution gives users (mail administrators) the most flexibility because you are running your own mail server but also takes more knowledge of mail servers and setup.