Message Transfer Agent

What Does Message Transfer Agent Mean?

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.

Techopedia Explains Message Transfer Agent

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:

  • Accepting messages originating from the user agent and forwarding them to their destination (other user agents)
  • Receiving all messages that are transmitted from other user agents for further transmission
  • Keeping track of each and every activity and analyzing and storing the recipient list to perform future routing functions
  • Sending auto-responses about nondelivery when a message does not reach its intended destination

The computing terms MX, mail exchanger and mail server may be used to refer to the system that is engaged in MTA functions.


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Margaret Rouse is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical, business audience. Over the past twenty years her explanations have appeared on TechTarget websites and she's been cited as an authority in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine and Discovery Magazine.Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages. If you have a suggestion for a new definition or how to improve a technical explanation, please email Margaret or contact her…