In computing, a server is a computer program or a device that provides functionality for other programs or devices, called “clients”. This architecture is called the client–server model, and a single overall computation is distributed across multiple processes or devices.
Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. An MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol.
The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with an MX record containing the domain name of the host(s) providing MTA services.