In most organizations, there is one machine which stores email for all users, even if the email originated from different machines. UNIX machines are multi-user. It is possible for a user to send email to other users on this machine. The default behavior is to store the message in the mail folder of the user on this machine. This is often inappropriate. UNIX machines are not only multi-user, but network oriented. This user (the recipient) may very well have accounts on many machines on the network (often on all machines) and may elect to read email messages on another machine. Having his email messages distributed onto different machines on the network is not useful.
Instead, specify the fully qualified name of the mail server of the organization, and all email messages will be forward to it.