A mail server is an application that receives email from email clients or other mail servers. It is the workhorse of the email system. A mail server usually consists of a storage area, a set of user definable rules, a list of users and a series of communication modules. The storage area is where mail is stored for local users, and where messages that are in transit to another destination are temporarily stored. It usually takes the form of a simple database of information. The user defined rules determine how the mail server should react when determining the destination of a specific message, or possibly react to the sender of the message. For example: specific email addresses can be barred, or certain users can be restricted to only sending messages within the company. The list of users is a database of user accounts that the mail server recognizes and will deal with locally.

The communications modules are the components that actually handle the transfer of messages to and from other mail servers and email clients. Depending upon the requirements of the mail server there may be a number of different modules installed for use. A person, sometimes called a Postmaster, maintains the mail server and the list of user accounts that it supports.

Most mail servers are designed to operate without any manual intervention during normal operation. They wait for a message to be sent to them and process it accordingly, or collect messages from other mail servers at predetermined intervals.