An MTA is a service application that handles the relay or forwarding of electronic mail messages to recipients or relay sites from senders. It may also store incoming emails.
MTAs are ideally created to receive electronic messages from other clients. Every message is expected to have that additional information under SMTP. The MTA includes a “received” tag at the top of the message header. This tag is necessary to ensure a consistent routing hierarchy for all messages.
If the recipient of a message is not hosted locally, the message gets routed to the next available MTA. MTA functions by backend operational activities while the user only interacts with the email client interface. This explains why MTA is relatively unknown to users.
The MTA accepts messages coming from the user agent and forwards them to their destination – other user agents. The MTA also accepts every message transmitted from other user agents for transmission to other agents. Likewise, the MTA helps to monitor activities, analysis, and storage of the recipient list to perform future routing functions.
Lastly, the MTA sends auto-responses to notify senders of non-delivery when a message doesn’t get to the intended inbox.
A system engaged in MTA functions may be referred to as a mail server, mail exchanger, or MX.