What Is A Mail Transfer Agent?

The Warmup Inbox Team
The Warmup Inbox Team

There are so many email terms out there, and it can get confusing. We totally get it, which is why we’re here to help.

It’s normal to scratch your head at what a Mail Transfer Agent (MTA) is because it’s often called numerous other things:

  • A mail server
  • Mail relay
  • Mail router
  • Internet mailer
  • Message transfer agent (MTA)
  • Mail transport agent (MTA)

Let’s clear the air once and for all: these terms refer to the same thing. For this particular article, we’ll call it a Mail Transfer Agent, or MTA.

Outside of this article, you can call it by whichever name you like best. But no matter its name, MTAs are a vital part of sending and receiving emails.

Mail Transfer Agent Definition

Mail Transfer Agents are the software in charge of getting emails from point A to point B. When you send an email, it’s given to an MTA to begin the delivery process.

Between the time you hit send and the email arrives in the recipient’s inbox, MTAs take care of everything, including:

  • Queueing
  • Throttling
  • Scheduling
  • Managing connections
  • Transferring data
  • Processing deferrals
  • Generating bounces
  • Tracking delivery status.

As you can see, MTAs stay pretty busy.

MTAs use Simple Mail Transfer Protocol (SMTP) to complete all of their tasks. SMTP works to transfer data as intended, and MTAs carry out the functions specified by the protocols.

MTAs work behind the scenes to ensure secure and efficient email delivery. Most users interact with an email marketing tool or their email provider rather than the MTA interface. But when MTAs are correctly configured, the deliverability and performance of your email campaigns can skyrocket.

How Do Mail Transfer Agents Work?

The MTA is a part of the chain of events that gets the message you send to the intended recipient.

When you hit send, the message is transferred from your Mail User Agent (MUA), your email client. Your MUA passes it along to the Mail or Message Submission Agent (MSA), then the MSA forwards the message to the MTA.

Now that the MTA has it, it begins passing the message like a baton to get it to the recipient’s Mail Delivery Agent (MDA). If you’re sending an email to a local server, the process is pretty quick. But if your recipient isn’t on a local server, then one MTA will pass the message along to another MTA until it reaches an MTA that can get to the MDA.

We know that’s incredibly confusing. Think of it as if you’re sending paper invitations to a party. Some of your guests live in the same town as you do, so the post office that accepts the invitation can bring these invites directly to the recipients.

But let’s say you’re sending an invitation to someone out of state. When your post office gets the invitation, they have to send it to a larger post office that can put it on an airmail carrier. Once it lands, it’s sent to another post office so your recipient’s local delivery person can put it in their mailbox.

Back to email sending, once the MDA gets it, it’s sent to the recipient’s MUA and is delivered.

Sender’s MUA -> MSA -> MTA -> (MTA -> MTA ->) MDA -> Recipient’s MUA

While the MTA has a message, it queries the MX records and chooses a mail server to transfer the emails. If an email can’t be delivered to its final destination, the MTA sends the automatic response to let the sender know.

Queuing Emails

Since MTAs are juggling a plethora of messages, they generally handle mail with a “store-and-forward” model. When they receive a message, they put it into a line and ping the recipient’s server. Once the server responds, the email is sent through.

MTAs will make multiple attempts to reach the server, but if they don’t get a response, they’ll send the message back to the sender.

How Do Mail Transfer Agents Affect Email Deliverability?

MTAs directly influence email deliverability because they can protect and strengthen your sender’s reputation. Numerous factors impact your deliverability rate, but your domain and IP reputation carry the most weight. That’s because as soon as a receiving server identifies a sender as unsafe, all future messages from them are sent right to the junk folder or bounced back.

MTAs help bolster your email reputation in multiple ways.

Warming Up Your IP Address

Especially if you’re sending out mass emails for marketing campaigns, your email needs to be warm before you start sending out your target number of messages.

MTAs are a significant component of IP warming because they help you route your test emails during the warming process. Your MTA can help you hit your daily targets of sent messages without sending out more messages than you should.

Configuring Sending Flows

Receiving domains set limits on incoming mail, and if the limit is exceeded, they may mistake the sending server as an untrustworthy source.

You can configure your MTA to limit sending when needed and avoid this problem. If the recipient’s domain rejects a message, your MTA will pause the email queue for that domain. After some time has passed, the MTA will resume sending messages from that queue at a slower pace until everything is back to normal.

This helps protect your sender’s reputation by stopping any unnecessary delivery failures.

Bypassing the Graylist

Sometimes receiving servers will use a graylist to protect their users from spam messages. Graylists use MTAs to temporarily reject any email from a sender the server doesn’t recognize.

When a message is graylisted, the recipient’s server sends a request to the sender’s server to attempt delivery again later. Legitimate servers will recognize this request and send the message again, and the recipient’s server will accept it.

MTAs help streamline this process by putting the returned message into a new queue and resending it when it’s safe to do so.

Additionally, MTAs can be used for setting up email throttling rules, routing guidelines, monitoring the flow of outgoing mail, and more.

How Are Mail Transfer Agents Used in Email Marketing?

In email marketing, MTAs are used to increase your deliverability rate. When you’re trying to reach a broad audience, you need as many tools as possible to ensure your emails get to your customer’s inboxes.

Like we always say, writing awesome emails is only half the battle; getting them in front of your customers is the main fight.

Before you begin warming up a new domain or IP address, you can install an MTA on your mail server to help queue your emails and guarantee smooth delivery.

  • Sendmail/Proofpoint: Proofpoint recently acquired Sendmail, but many people still know the MTA by its OG name. As one of the oldest MTAs, Sendmail is old reliable.
  • Postfix: Postfix provides an easy-to-use platform that also enables junk mail control, multiple protocols, database and mailbox support, and address manipulation.
  • Exim: Exim is free and flexible and has utilities like Lemonade to enable mobile messaging and more. Its' list of configurations includes access control lists, content scanning, encryption, and routing controls.
  • Qmail: This MTA is also free, and Qmail is tiny but mighty. It packs a punch while being reliable, secure, and efficient.
  • Mutt: Mutt is a terminal-based client for Unix-style operating systems. Some of its features include message threading, support for multiple mailbox formats, delivery status support, and multiple message tagging.
  • Alpine: Based on the Pine messaging system, Alpine is a great option for starters because it’s easy to navigate. The terminal-based email client is highly customizable through the setup command platform.
  • OpenSMTP: OpenSMTP is an open-source MTA that comes with a web service that allows sending emails through an HTTP webserver. It’s primarily used for local deliveries or relaying messages to other SMTP servers.

For more tips and tricks to increase your deliverability, visit our blog.