Kaz Kylheku put forth on 9/14/2010 7:25 PM:
Correct my misunderstanding, but mail routing would be if the "university" mail server also accepted mail for the "cs.university" domain, and passed it on to the right server, no?
There is only one domain, university.edu. cs.university.edu is a host on the network, not a sub domain of university.edu. All mail is sent to user@university.edu. What the mailhub does is look at the user tables to determine if a mailbox is local or on another university host. In the latter case, there will be an entry in /etc/aliases for each user address that needs to be delivered to another host. Here's a link detailing how to do this in Postfix:
http://www.postfix.org/STANDARD_CONFIGURATION_README.html#local_network
Wietse uses the word "forward" in his how-to which I don't agree with. In a purely technical mail delivery sense, the two words "routing" and "forwarding" are interchangeable. But in the practical sense, the internet layperson has a distinct notion of what "forwarding" an email is, and the same layperson will have never heard the term "mail routing". This is why I use these words distinctly and definitively.
Forwarding is: user@gmail.com to other-user@yahoo.com
Routing is: bob.smith@university.edu to bob.smith@cs.university.edu
In the case of forwarding, mail is being sent from one account at one domain to another account at another domain.
In the case of routing, mail is being received for a user at a domain mailhub host and routed to another host in the domain where his mailbox is stored. When this user sends mail to user@gmail.com, it is submitted to cs.university.edu which then relays it to the mailhub host, which then relays it to the destination domain. When this user sends an email to jane.doe@university.edu, who is not in the CS department, the MTA on cs.university.edu looks up the alias for jane.doe which points to history.university.edu and routes the email to her mailbox on the history dept server. These examples are not "forwarding" of email, but "routing" of email.