A Mail Transfer Agent (MTA) is software that transfers email messages between mail servers. When an email is sent, the MTA accepts the message from the sending system, identifies the recipient's mail server, and transfers the message to that server.
For example, when a business sends an email to a customer, the MTA uses the recipient's domain to identify the appropriate mail server and attempts to deliver the message. If the receiving server is temporarily unavailable, the MTA can place the message in a queue and retry delivery later.
What Does an MTA Do?
An MTA manages the transfer of email between mail servers. Its functions include:
- Accepting outgoing messages from an email client, application, or other mail system.
- Identifying the recipient's mail server by looking up the domain's Mail Exchange (MX) records.
- Transferring messages to the receiving mail server using SMTP.
- Queuing messages when temporary delivery problems prevent immediate delivery.
- Retrying delivery when the receiving server is temporarily unavailable.
- Reporting permanent delivery failures when a message cannot be delivered.
MTA vs. Email Client
An MTA and an email client perform different functions. An email client is the application a person uses to compose, send, receive, and read email. An MTA handles the transfer of email between mail servers.
For example, when a user clicks Send in an email application, the email client passes the message to the mail server. The MTA then handles the transfer of the message to the recipient's mail server.
Why Is an MTA Important for Email?
An MTA handles the actual transfer of email between mail servers. Without an MTA, a sending system would not have the service responsible for establishing the connection with the recipient's mail server, transferring the message, and managing delivery when the receiving server is temporarily unavailable.