Understanding Message Transfer Agents (MTAs): The Backbone of Email Communication

  • May 31, 2023


In today's digital age, email has become an integral part of our personal and professional lives. We rely on email for communication, collaboration, and information exchange. Behind the scenes, a complex system of technologies and protocols ensures the smooth delivery of emails. One such crucial component is the Message Transfer Agent (MTA), sometimes also called a Mail Transfer Agent. In this blog post, we will delve into what an MTA is, its role in the email delivery process, and its significance for businesses and individuals alike.

What is a Mail Transfer Agent (MTA)?

A Mail Transfer Agent, also known as a Mail Server, is a software application responsible for sending, receiving, and routing emails over the Internet. When you hit the "Send" button on your email client, your email client relays the message via SMTP (Simple Mail Transfer Protocol) to the MTA, which takes over and acts as a postman, ensuring your email reaches the intended recipient's mailbox. It's the behind-the-scenes workhorse of email delivery, working tirelessly to ensure the smooth and efficient transfer of messages across various servers.

The Role of an MTA in Email Infrastructure

Without MTAs, there would be no email, here's just a few of the roles an MTA performs:

Receiving Messages

Whether you're sending a message or receiving one, a remote MTA or your email client (Message User Agent or MUA) connects to the MTA through a series of handshakes, the MTA verifies the recipient's address, and queues the email. It may also employ techniques such as DNS lookups and reverse DNS checks to ensure the authenticity of the sender and reduce the likelihood of spam.

Queueing Messages 

In life, there are no guarantees, so an MTA needs to be able to queue messages so it can operate on a Store and Forward basis. There are many reasons why a message can't be delivered on the first attempt, from the remote MTA being out of service or too busy to accept new messages, to the Mailbox Provider (MBP) using GreyListing tactics, where they temporarily fail incoming messages to see whether your MTA will retry the message later (automated spambots rarely retry any given message, preferring to move on to the next recipient before they are detected).


In addition to sending and receiving emails, MTAs play a crucial role in routing messages across different email servers. When the recipient's server is not directly reachable, the MTA will look for intermediate MTAs, known as Mail Exchange (MX) servers, to relay the email until it reaches its destination. Additionally, most MTAs operate on both an inbound and outbound basis, requiring them to route messages from the outside world to antispam and antivirus systems, then onward to a mailbox server (Message Delivery Agent or MDA) for later access.

Sending Messages 

Once an MTA has received a message, validated it, and queued it, it needs to be able to send that message to the next stop in its journey. The MTA does this by looking up a special DNS record called an MX record, which tells it where to find the MTA(s) responsible for routing messages for that domain. Your MTA then connects to one of the servers listed in the MX record and attempts to deliver the message.


MTAs also contribute to email security. They can implement various security measures like Transport Layer Security (TLS) encryption, authentication mechanisms (SPF, DKIM, DMARC), and spam filtering to protect against unauthorized access, data breaches, and malicious content.

Importance of MTAs for Businesses

Reliable Communication

Many businesses depend on MTAs to ensure that emails are reliably delivered, enabling seamless communication between individuals and businesses. They handle high volumes of emails efficiently, making sure that important messages reach their recipients without delay.

Brand Reputation

For businesses, MTAs are essential in maintaining a positive brand reputation. By implementing proper authentication mechanisms and spam filtering, MTAs help prevent email spoofing and phishing attacks. This ensures that emails sent from a business domain are genuine, increasing trust and reducing the chances of being marked as spam.


MTAs are designed to handle large-scale email operations, making them suitable for businesses of all sizes. Whether you're a small startup or a multinational corporation, MTAs provide the infrastructure needed to manage email traffic effectively.

Compliance and Legal Requirements

Certain industries, such as healthcare and finance, have strict compliance and legal requirements for email communication. MTAs can be configured to meet these requirements, such as encryption for sensitive data transmission, ensuring businesses remain compliant with regulations.

  • Should You Operate Your Own MTA Infrastructure?

  • As with most important business questions, the answer is "it depends".
  • With the growth of SaaS, there are many offerings out there for cloud MTA services that will handle the availability and proper configuration of your sending infrastructure at a reasonable cost for lower-volume senders, a small sample of which includes:
  • SendGrid
  • MailGun
  • PostMark
  • PepiPost
  • When your message volume is in the thousands of messages per day, it's more cost-effective to use such a service to handle your email infrastructure because the pool of email infrastructure experts is small, and there is a lot to keep track of to ensure your messages make it to the inbox.
  • As your sending volumes grow to millions of messages per month, the economics of email infrastructure start to shift: eventually you can start seeing costs of thousands and even tens of thousands of dollars per month to use a cloud email service and at that point the economics of hosting and managing your own MTA start to make sense.
  • That doesn't mean you can't take advantage of the cloud, many large-volume senders host their MTA servers in a public cloud such as AWS and Azure, and even take advantage of consulting services to manage their MTA servers. The largest cloud senders can literally save millions of dollars per year by moving from SendGrid to their own MTAs.
  • How to Choose an MTA

  • When it's time to migrate to your own MTA, there are different options you can choose:
  • Commercial MTA

  • PowerMTA
  • Over the years there have been a number of commercial MTA options to choose from, including Momentum, PowerMTA, MailerQ, Halon, and Green Arrow. Commercial MTAs are generally high-performance, with modern authentication functionality and commercial support for mission-critical email infrastructure. While some vendors still provide perpetual license options (with an annual support and maintenance fee), most commercial MTA vendors are now charging annual license fees based either on server count or message volume.
  • Open Source MTA Projects

  • Open Source MTA
  • In addition to the legacy projects such as Sendmail, Postfix, and Exim, there are more modern MTA projects such as Haraka, ZoneMTA, and Postal. These projects generally are lower performance than their commercial counterparts and lack the support offerings of commercial vendors, but as Open-Source solutions they are free-to-use and there is no risk of lost functionality if the developers cease operation.
  • Commercially Produced Open Source

  • Open Source MTA
  • KumoMTA provides the best of both worlds: developed by a commercial team of experienced email infrastructure professionals, KumoMTA has the performance and flexibility of a commercial MTA, along with the available support SLA that businesses need, while being released under an Open Source license that keeps cost and risk at a minimum.
  • Summary

  • Mail Transfer Agents (MTAs) are the unsung heroes behind the scenes of email communication. They enable reliable delivery of emails, implement security measures, and play a crucial role in maintaining brand reputation for businesses. Understanding the role of MTAs helps individuals and organizations appreciate the complexity involved in email delivery and make informed decisions about the email infrastructure they use.