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Mail Transfer Agent: What Is MTA Exactly & How Does It Work?

Updated: Mar 29

There are so many email terms out there, and it can get confusing. We totally get it.


That is why we’re here to help.


This time, we are going to take a look at the term MTA - or alternatively known as a mail server, mail relay, mail router, mail transport agent, etc., etc. (You can see the problem here, right?).


Since MTAs play a critical role in sending and receiving emails, we want to clear the air once and for all and explain this concept as simply as possible.



Mail Transfer Agent

What Is a Mail Transfer Agent (MTA)?


MTA - Mail Transfer Agent (or Message Transfer Agent) is the software in charge of getting emails from point A to point B. When you send an email, the Mail Transfer Agent takes over the task of delivery.


The MTA takes over everything that happens between sending and delivering the email into the recipient's inbox - this includes several operations in the process, such as:


  • Queueing: Storing emails in a line-up or queue until they can be sent to the recipient's server.

  • Throttling: Controlling the rate at which emails are sent to avoid flooding the recipient's server and getting marked as spam.

  • Scheduling: Determining the most optimal time to send out emails based on various factors, including the recipient's time zone.

  • Managing connections: Controlling the establishment, maintenance, and termination of network connections used for email transmission.

  • Transferring data: Moving the actual email data (content, attachments, etc.) from one point to another over the network.

  • Processing deferrals: Handling temporary failures in email delivery, such as when the recipient's server is busy or down.

  • Generating bounces: Creating and sending bounce messages back to the sender when emails cannot be delivered.

  • Tracking delivery status: Monitoring and recording the status of each email sent, whether it's delivered, in the queue, bounced, or deferred.



MTAs use SMTP (Simple Mail Transfer Protocol) - a set of rules computers use to send and receive emails in order to complete their tasks, enabling data transfer as per the protocol's guidelines.


Users generally interact with their email clients or email providers rather than directly with the MTA.


However, a correctly configured MTA can significantly improve the deliverability and performance of your email campaigns.


How Do Mail Transfer Agents Work?


When you send an email, it's like passing a baton in a relay race.


Your email client - Mail User Agent (MUA), which is like the first runner, hands off the email to the Mail Submission Agent (MSA).


The MSA, our second runner, then gives the email to the MTA.


If your email is going to someone nearby (on a local server), the MTA can easily pass it to the final runner, the Mail Delivery Agent (MDA), which delivers it to the recipient's inbox.


Alternatively, if your email is going to someone far away (on a non-local server), the MTA may need to hand it off to other MTAs along the way, like extra runners in the race:

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

While carrying the email, the MTA checks MX records to pick the right server for passing along the email.


If, for some reason, the email can't be delivered, the MTA lets you know by sending an automatic response, just like a runner would signal if they can't finish the race.


Queuing & Delivery of Emails


MTAs handle emails based on a ‘store-and-forward’ model.


Upon receiving a message, they place it in a queue and ping the recipient’s server. If the server doesn’t respond, the MTA makes multiple attempts to reach it.


If all attempts fail, the MTA sends the message back to the sender.


How Do Mail Transfer Agents Affect Email Deliverability?


MTAs play a significant role in influencing email deliverability by safeguarding and enhancing your sender's reputation.


The deliverability rate is heavily influenced by aspects such as your domain and IP reputation.


Here are some ways how MTAs assist in maintaining your email reputation:


  • IP warming: When sending out bulk emails for marketing campaigns, your email address needs to be ‘warm’ before you start sending out your target number of messages. MTAs assist in this by routing your test emails during the IP warming process. This process helps you hit your daily targets without exceeding limits.

  • Configuring sending flows: Your MTA can help manage the email sending rate to respect these limits. If a message is rejected by the recipient's domain, your MTA will pause the email queue for that domain. It will then resume sending messages from that queue at a slower pace, preventing unnecessary delivery failures.


In addition to these primary functions, MTAs also perform several other tasks:


  • Bypassing graylists: MTAs help streamline the process of bypassing graylists, which temporarily reject any email from unrecognized senders. Once safe, the MTA resends the returned message.

  • Setting up email throttling rules: These are rules that limit the number of email messages sent out in a specific timeframe, preventing your server from being perceived as a spam source.

  • Routing guidelines: MTAs can establish guidelines to control the path that an email takes from the sender to the recipient.

  • Monitoring the flow of outgoing mail: MTAs track all outgoing emails, providing insight into email performance and identifying potential issues.

If you have ice-cold email addresses and lackluster sender reputation, you might consider tools like Warmup Inbox - it is capable of properly warmup your inboxes before launching your email campaigns, keeping your email domains out of blacklists, and improving your overall email deliverability rates. You can sign-up and try Warmup Inbox completely for free (no credit card needed)!

How Are Mail Transfer Agents Used in Email Marketing?


In email marketing, the role of Mail Transfer Agents (MTAs) extends beyond just sending and receiving emails.


They can be configured and fine-tuned to optimize your email campaigns, maximize reach, and improve engagement rates.


Before diving into the process of launching a new marketing campaign or warming up a new domain or IP address, installing an MTA on your mail server can pave the way for smooth email delivery.


This careful orchestration of your email dispatch can increase the likelihood of your messages landing in the recipient's inbox rather than being trapped in spam filters or bouncing back.


Take a look at how MTAs contribute to different stages of an email marketing campaign:


  • Planning stage: MTAs can help determine the rate at which emails are sent out, the number of retries for each email, and the duration between each retry. This allows for a balanced load on your server and prevents your emails from being marked as spam.

  • Execution stage: In the throes of an email campaign, MTAs manage the actual delivery of thousands, or even millions, of emails. They handle the process of connecting to various recipient servers, sending out emails, and then appropriately disconnecting.

  • Analysis stage: MTAs aren't just about sending emails – they're also about gleaning insights from them. They can provide useful data such as the number of emails delivered, the number of bounces, and even the time it takes for an email to be delivered. These insights can help optimize future campaigns.

  • Customer Relationship Management (CRM) integration: MTAs can be integrated with your CRM system to streamline your email marketing process. For instance, you can automatically trigger personalized emails based on specific actions or milestones in the customer journey.

  • Scalability: MTAs are designed to manage massive volumes of emails, making them a valuable asset for any growing business. As your customer base expands, your MTA can scale alongside it, keeping pace with your growth.


How To Select Mail Transfer Agent?


Choosing the right MTA for your organization can be quite a task given the multitude of options available.


Here are a few aspects to keep in mind:


  • The volume of email traffic: If your organization sends and receives a high volume of emails, you would require an MTA that can handle this load efficiently without any delays or loss of messages.

  • Integration with existing infrastructure: If you have an existing email infrastructure in place, it's essential that the MTA you choose can integrate seamlessly with it.

  • Security features: As mentioned earlier, an MTA with built-in security features would be a better choice to protect your organization from potential email attacks.

  • Customizability and scalability: An MTA should also be customizable to your organization's specific needs and scalable to grow with your business.

  • Budget: Lastly, your budget plays an essential role in your choice of MTA. Some MTAs are free, while others come with a cost.


The Most Popular Mail Transfer Agents


There are numerous MTAs available today. Here are some popular ones:


  • Sendmail/Proofpoint: Although Proofpoint acquired Sendmail, many people still know the MTA by its OG name. As one of the oldest MTAs, Sendmail prooved its reliability over time.

  • Postfix: This MTA offers an easy-to-use platform with features like junk mail control, multiple protocols, database, and mailbox support, and address manipulation.

  • Exim: Exim is a free, flexible MTA with features like mobile messaging through utilities like Lemonade. It supports access control lists, content scanning, encryption, and routing controls.

  • OpenSMTP: OpenSMTP is an open-source MTA with a web service for sending emails through an HTTP web server. It is primarily used for local deliveries or relaying messages to other SMTP servers.

  • 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.

  • Qmail: This MTA is also free, and Qmail is tiny but mighty. It packs a punch while being reliable, secure, and efficient.


Mastering email marketing is only half the battle; ensuring your messages reach the customers is the main fight.


With a properly configured MTA, you can make the most of your email marketing efforts and ensure that your messages make it to their intended inboxes.


Conclusion: Mail Transfer Agents Are Simply Necessary


MTAs are indispensable tools when it comes to electronic mail messages.


They act as the backbone for delivering emails from the sender to the recipient, taking care of a variety of tasks, including queueing, throttling, managing connections, transferring data, and generating bounces.


Without MTAs, the seemingly simple task of sending and receiving emails would be a complex and laborious process.


The email landscape is constantly changing, with new technologies and practices continually emerging.


But one constant remains: the crucial role that Mail Transfer Agents play.


As long as we rely on email as a means of communication, the importance of MTAs will continue to be paramount.


As such, understanding how MTAs work and how to use them effectively is vital for anyone dealing with email, whether you're a business owner, a marketer, or an individual user.

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