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A message transfer agent (MTA) is a software application used within an Internet message handling system (MHS). It is responsible for transferring and routing an electronic mail message from the sender’s computer to the recipient’s computer. The basic platform for an MTA is an exchange system with client/server architecture.
A message transfer agent receives incoming emails and forwards the messages to individual clients/users. The main function of the MTA is forwarding the incoming message to the proper end-user or destination. Microsoft Exchange and UNIX sendmail are the best examples of MTAs.
MTA is also known as mail relay.
Normally, MTAs receive electronic messages from other clients. All messages must have the extra information required under SMTP. The MTA adds a “received” tag at the top of the message header to maintain the routing hierarchy for all messages. When a recipient of a message is not hosted locally, then the message will automatically be routed to the next MTA.
The MTA’s operational activities are always performed on the back end, but a user is unaware of these because he or she will interact with an email client interface.
The major functions of an MTA are:
The computing terms MX, mail exchanger and mail server may be used to refer to the system that is engaged in MTA functions.