What’s a transport agent and how does it work?

| 18th Nov 2021

A transport agent routes email messages between users and also manages deliveries. In other words, it's an application that receives incoming emails and routes outgoing emails. For example, transport agents act as servers when receiving email messages and then as clients when routing emails to recipients. Each transport agent performs some or all of the tasks in this process.  

One way to understand the stages of an email is to think of the journey a physical letter takes through the Royal Mail. The transport agent's tasks are roughly equivalent to the stages between a letter (collected from a postbox) arriving at the nearest sorting office and the letter going out in the postman or post woman's bag.

Although most users don't directly encounter a message transport agent (MTA), it's vital to the email process. Using transport agents in a business email context can greatly improve both reliability and security.

What is a transport agent?

Part of the confusion over the term "mail transport agent" is that several other terms cover the same or similar concepts. These include the mail router, mail relay, and mail server.

Another reason for the confusion is that a transport agent can describe a specific stage of the overall email process and a specific tool performing some or all tasks in this stage.

To make things a bit clearer, let's break down each stage of an email:

  1. The sender uses Mail User Agent software to create an email and begin the sending process.
  2. A Mail Submission Agent accepts the message from the Mail User Agent and adds it to the queue of a Mail Transport Agent. (Sometimes, the submission agent and transport agent are built into the same software.)
  3. The Mail Transport Agent works through a queue of messages and directs each one to its intended location. (Sometimes, a message passes through multiple transport agents in the mail relay.)
  4. If the message gets through successfully, a Mail Delivery Agent takes it through the final step to the recipient's account. If the message isn't delivered, the Mail Transport Agent returns a non-delivery notification to the sender.

How transport agents work

The primary task of most transport agents is identifying the recipient and routing the message. That involves identifying the mail server (or servers) that receives messages for a particular domain name. These are listed through mail exchanger (MX) records, which are part of the same Domain Name System used to "translate" website addresses into the servers that physically house a web page.

Transport agents also handle queueing to send messages most efficiently. That's particularly important for messages with multiple recipients, such as in email marketing. The most common method is to hold outgoing messages in a buffer, send them one at a time, then resend any undelivered ones at set intervals. 

This process helps overcome temporary barriers to delivery (such as the recipient's mailbox being full or the message being held up by a spam filter), maximising the chances of getting the message through without retrying too frequently and clogging up the system. Depending on the settings for retrying delivery, the transport agent will eventually stop the process if still unsuccessful and return a non-delivery message to the sender.

Exchange transport agent

While many transport agents are designed to cover the entire direction and relaying process, Microsoft Exchange allows multiple transport agents to perform a specific task. That could include combating spam, routing a message based on a particular characteristic, or logging message routing to ensure the integrity of messages. 

In this context, each transport agent is a specific DLL code that performs the task in question. A single message will pass through multiple transport agents on its journey.

Benefits of using Egress Transport Exchange Agent

As well as these basic functions, some transport agents offer additional features or suit specific needs. Egress Transport Exchange Agent is custom-built to:

  • Integrate with Microsoft Exchange.
  • Give you greater control over how messages are routed within your organization.
  • Perform security tasks, for example, to reduce the risk of phishing attacks that pose as internal messages.
  • Work on both hub transport and edge servers (which commonly handle internal and external email, respectively.)


What is the purpose of a mail transport agent?

To route email messages to the correct recipient server and return a non-delivery message if needed. (Some transport agents are dedicated to specific tasks that aid this process or use data from it.)

How does a mail transfer agent work?

A transport agent works by:

  • querying MX records (part of DNS) to find the correct server
  • queuing multiple messages
  • routing each message as needed
  • retrying undelivered messages on a set schedule
  • returning a non-delivery message after a set time or number of attempts

Is Outlook a mail transfer agent?

Outlook is primarily a Mail User Agent, meaning software that creates emails, starts the sending process, and displays emails for the recipient to read. In most cases, Outlook performs tasks before and after a Mail Transfer Agent has done its work.