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.