Mail Transfer Agent (MTA)

Joran Hofman
April 4, 2021

What is a Mail Transfer Agent?

A message transfer agent (MTA) is a software application or computer programused within an Internet message handling system (MHS) that transmits emails between the computer equipment of a person who sends and another receiving.

It is responsible for the transfer and route of an email message from the source computer to the arrival computer. The main support of an MTA is an exchange system with client/server architecture.

An MTA is also known as a mail relay.

What is an MTA (Mail Transfer Agent) used for?

An MTA is responsible for the main tasks concerning email delivery; these functions include: 

  • The limitation of messages received and sent,
  • The tails,
  • The administration of the necessary connections,
  • The programing,
  • The generation of rebounds,
  • Data transfer,
  • The processing of deferrals, and
  • Track the delivery status of messages.

An MTA also includes features for handling incoming email, such as replies to be sent and messages bounced remotely.

How the MTA (Mail Transfer Agent) works?

An MTA is just one part of the entire email delivery process. It gets an email from the mail delivery agent, who, in turn, receives it from the mail user agent (MUA). The MUA is commonly known as an email client, a program or software that is used to handle email-related matters.

When the MTA receives the email, the relay kicks in.  The email can be forwarded to other MTAs if the recipient is not hosted locally. Then it is the turn of the mail delivery agent (MDA). This is the last stage of the email before it is delivered to the recipient's mailbox. The sending of email is done through SMTP (or extended SMTP), and for the last stage (MDA to MUA), POP3 or IMAP4 is used.


In summary, MTAs do the following: 

  • Accept emails sent by mail user agents.
  • Check MX records and select a mail server to transfer emails.
  • Send automatic reply messages if an email has not reached the destination.

In most cases, MTAs employ a store-and-forward form of mail management, which means that outgoing mail is queued and waits for a response from the receiving server. An MTA will try to send emails repeatedly. If the mail is not delivered within the established period, it will be returned to the mail client.

Cloud-based SMTP relay vs. local MTA.

It cannot be said that on-premises MTAs are entirely better than any cloud-based email infrastructure, as each specific project has its own unique needs and can benefit from a cloud-based solution.

An MTA in local facilities.

On-site mail transfer agents are the primary choice for businesses and large businesses. This brings with it a complete email infrastructure that can be configured according to requirements.


  • All aspects of email sending conformations can be addressed.
  • They have higher reliability.
  • They facilitate integration with internal tools and software.
  • They offer connection to the email marketing infrastructure with internal information sources.
  • There are almost no API limitations.
  • It can send a large number of emails without lag or some other speed issue.
  • Offers full control over email sending settings.


  • Local MTAs are expensive.
  • Space is required to configure hardware.
  • The setup of the entire system takes a long time (an average estimate of 3 months).
  • The system owner assumes full responsibility for  administering the infrastructure and security aspects of the email database.
  • It is difficult to scale up.

A cloud-based SMTP relay service.

When you don't have a significant amount of financial resources and your email sending needs are moderate, then you can consider the option of a cloud-based SMTP relay service (Mailgun or SendGrid). It's a quick and helpful answer to get started. The main disadvantage is that it is a shared service. You do not have control of everything, and the infrastructure is shared with others (although some services have dedicated IP optionally).


  • It is easy to set up and use.
  • It is much cheaper than local MTAs.
  • They have a great variety of prices.
  • Security is provided by the SMTP relay service.
  • It is easy to scale according to the needs of each company.


  • In the long run, it can be costly.
  • Integrations supported by the SMTP relay service are required.
  • It doesn't offer much control.

Mail Transfer Agents examples.

  • Exim.
  • Postfix.
  • Sendmail.
  • Qmail.
  • Microsoft Exchange.
  • UNIX Sendmail.

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