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 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.

Email server is also called as Message Transfer Agent(MTA). This MTA transfers electronic mail messages from one computer to another computer using client–server application architecture. An MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol.

A MTA receives mail from either another MTA, a Mail Submission Agent (MSA), or a Mail User Agent (MUA). The transmission details are specified by the Simple Mail Transfer Protocol (SMTP). When a recipient mailbox of a message is not hosted locally, the message is relayed, that is, forwarded to another MTA. Every time an MTA receives an email message, it adds a Received trace header field to the top of the header of the message, thereby building a sequential record of MTAs handling the message. The process of choosing a target MTA for the next hop is also described in SMTP, but can usually be overridden by configuring the MTA software with specific routes.

For this purpose the MTA transfers the message to the message handling service component of the message delivery agent. Upon final delivery, the Return-Path field is added to the envelope to record the return path.