Often referred to as simply "mail server", an e-mail server is a computer within your network that works as your virtual post office. A mail server usually consists of a storage area where where e-mail is stored for local users, a set of user definable rules which determine how the mail server should react to the destination of a specific message, a database of user accounts that the mail server recognizes and will deal with locally, and communications modules which are the components that actually handle the transfer of messages to and from other mail servers and email clients.
Generally the person(s) responsible for the maintenance of the e-mail server (editing users, monitoring system activity) are referred to as the postmaster. Most mail servers are designed to operate without any manual intervention during normal operation.