A mail server , or mail hub , is a central server to handle all email traffic to and from a local network. Workstations on the network route all mail through the hub rather than routing it directly to the recipient. The hub rewrites the header of all of the outgoing messages so that they appear to come directly from the hub. All mail is addressed to one machine, so that outside users do not have to be concerned about multiple email addresses for a given site.
All mail is spooled centrally, making management easier.
This adds an extra level of security. Any changes in the mail configuration need only be done on the hub. The hub needs to know about all of the users on the network. This complicates account administration. There can be performance problems.