More than one machine receives e-mail. There are currently over 15000 userids which are able to receive e-mail at this university. There are over 2000 different machines that these people can use to read their mail. This is too much of a load for one machine to handle efficiently, thus some departments have elected to spool their own e-mail on their own servers. Since more than one machine receives e-mail, a mechanism must be in place to allow messages to reach these machines.
People move around all the time, especially the students.
Sometimes they want their mail sent to them in the CS department, sometimes they want it in the EE department. These changes should be transparent to the other users. It is unreasonable to require that people consult a campus directory every time they want to send a message. This statement has substantial implications. This assumption is certainly not correct for this environment since it requires that every machine on the campus maintain a mailbox for every user on the campus.