Mail Transfer Agent, The Key to Reliable Email Delivery

Mail Transfer Agent: What It Is and Why It’s Crucial for Email Delivery
Vitalii Piddubnyi Vitalii Piddubnyi 22 january 2025, 13:30 165
For beginners

The background work of sending one or a bunch of emails is not as simple as one might think. This nearly instant procedure requires a solid system that will take on the technical side. The Mail Transfer Agent, or MTA, is the solution that handles everything from address lookups and spam filtering to IP warm-up and message queue control. Maybe you’ve wondered why some emails arrive in your inbox within seconds while others are flagged as spam or delayed by hours. The answer frequently refers to how well your message transfer agent is configured.

In this article we will unwrap the concept of email MTAs and peculiarities of their work. We’ll look at how they function, why they’re important, and how to find the best one for your organisation.

What is a Mail Transfer Agent?

A Mail Transfer Agent is also referred to as message transfer agent, mail exchanger, or MX host. It is software that facilitates the transmission of email messages from one computer to another. Whenever you click the "send" button, it unfolds a chain of events right after this action. First, the email client (also called a Mail User Agent, or MUA) hands off the message to its designated MTA. The MTA then routes the message across the internet or local network using the Simple Mail Transfer Protocol (SMTP), transferring it to the Mail Delivery Agent (MDA) on the recipient's end. 

Essentially, without these programs, the message would never leave your or your organization's computers. Let's move on to see the peculiarities of an MTA’s work.

How do Mail Transfer Agents Operate?

To grasp the concept of MTA in more detail, we need to walk through the typical workflow of an MTA, which processes and routes email. Understanding all the details is essential, as even minor gaps may influence your email performance.

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Email Submission and Relaying

An MTA first accepts an email from a mail user agent (like Gmail's web client, Outlook, or a mobile application). This submission process happens over SMTP using a designated port, usually ports 25, 465, 587 or 2525. Besides the port number, the client needs to know the MTA’s host name, which is provided during initial setup.

Email client programs are not the only source of emails for a particular MTA. Alternatively, it may receive them from another MTA, to be transferred somewhere else. This process is called email relaying.

Lookup and Routing

First of all, the MTA consults Domain Name System (DNS), specifically Mail Exchange (MX) records, to determine which server is responsible for handling email for the recipient's domain. For instance, if your email is intended for someone@example.com, the MTA checks the DNS for example.com to find the mail server's address.

By the protocol rules, if a domain does not have any MX records, type A or AAAA record will be used instead. 

Connecting to the Recipient’s Server

Once the addressee’s mail server is identified, your MTA establishes an SMTP connection to deliver the email. Depending on authentication and encryption requirements (like STARTTLS for secure transmission), the MTA will handle all necessary connection steps.

Delivery Confirmation

Upon handing off the message, the recipient's MTA either accepts or rejects the email. Being accepted means the email should eventually arrive in the recipient's mailbox (although it’s yet undecided whether it will go to their inbox, spam folder, or any special folder, based on spam checks and user-defined filtering). Rejection code usually comes with a reason, like an invalid recipient, suspected spam, or policy violation.

The MTA keeps track of all these important events by storing them in mail logs.

Queue Management and Retries

Sometimes, the receiving server might be temporarily unavailable or busy. The MTA then queues the email and retries delivery at set intervals until either it succeeds or a timeout or permanent failure occurs.

The above are the main nuances of MTA performance. However, there are some other valuable features that we will consider further.

Beyond Mail Transfer: What Else MTAs Usually Do

A modern message transfer agent should offer advanced capabilities besides its main sending and receiving functions. Today, a good MTA can manage your sending rates to preserve IP reputation and utilize authentication protocols to combat spoofing. In other words, it will act as both traffic controller and guardian, maximizing deliverability. As to the advanced capabilities, let’s distinguish the essential ones.

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Managing Sending Rates

This vital function impacts deliverability, and some also call it throttling or rate limiting. When an organization or email provider sends emails in large volumes, the number of messages by itself may trigger spam filters. This is why the advanced MTAs can:

  • Limit the number of messages sent per second or per hour,
  • Automatically adapt to feedback from receiving servers.

Such actions help protect the sender's reputation.

Email Authentication

Email authentication impacts relationships with ISPs, builds trust, and helps the messages avoid ending up in a spam folder. Being optional previously, it now becomes a requirement enforced by major mailbox providers. MTAs should support the following authentication technologies:

  • Sender Policy Framework (SPF). Ensures that mail servers that are permitted to send on behalf of a domain are listed in DNS.
  • DomainKeys Identified Mail (DKIM). Uses digital signatures to verify that an email was indeed sent by a particular domain, and hasn't been tampered with.
  • Domain-based Message Authentication, Reporting, and Conformance (DMARC). Allows domain owners to define how emails that fail SPF or DKIM checks should be handled. If there's an issue, it will flag it and send reports.

Efficient Routing

Routing is a fundamental part of an MTA's functions. The mail transfer agent directs messages between the sender and the recipient’s mail servers. However, advanced email MTAs will do a little extra:

  • Route emails for specific domains through intermediate servers, if necessary.
  • Uses different relays (MTA-to-MTA) based on region or IP reputation.
  • Implements "smart hosts," which forward mail through specific gateways.

Efficient routing reduces delay, costs, and probable deliverability concerns.

Filtering Spam

A reliable MTA incorporates spam filtering mechanisms or integrates seamlessly with external filters. These security measures help prevent malicious messages. It may also perform reputation checks or query known spammer databases.

Warming Up IP Addresses

If you're sending large-scale email campaigns from a new IP address, you need an IP warm-up plan, starting with a small number of messages and gradually increasing it. Your MTA will help you progressively ramp up sending to build a good reputation with inbox providers.

If the MTA has all the above capabilities, your emails are in the right hands. The next step is to decide between software market leaders.

Making the right choice between MTAs isn't just about sending emails from A to B. Eventually, it is influenced by many aspects, from the vendor’s reputation to the setup process. Below, we will explore the best options available, their features, and the difference between on-premise MTAs and cloud-based SMTP solutions.

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Open source

In the world of Internet servers, free and open source software is not just a valid contender, but often the most time-proven and widely adopted solution. This is particularly true for open source MTAs, some of which boast a decades-long history of successful usage.

Postfix. It is a very popular message transfer agent known for robust security features and straightforward configuration. It was developed to be flexible and expandable, making it appropriate for various email delivery applications. Its key traits include:

  • Safe and resilient architecture.
  • Flexible queue management, simple configuration syntax, active user community.
  • Ideal solution for businesses or developers working with Linux systems.

Exim. It's customizable, with comprehensive policy settings for routing and email filtering. The main features are:

  • Extensive logging and troubleshooting options.
  • Robust spam control.
  • Effective usage of resources with minimum overhead.
  • Suitable solution for those who need advanced routing or specialized filtering rules.

Sendmail. Formerly known as Proofpoint, it is one of the oldest and most well-known MTAs. However, it may be considered a bit more complex than Exim or Postfix. Nevertheless, it is valued for:

  • High adaptability, large user base, long history.
  • Superior security features including spam, phishing, and virus prevention.
  • Comprehensive support for advanced email routing and delivery.
  • Ideal for legacy environments or administrators already well-versed in Sendmail's configuration files.

Proprietary

If you rely on a one-vendor infrastructure, a proprietary solution will often be your only option, and a quite expensive one too. However, paid MTAs are no less viable, and may offer features and support options not available with free software.

Microsoft Exchange Server. An excellent option for enterprises using Windows infrastructure. Well-integrated with Active Directory and Microsoft 365 services, its main features include:

  • Calendar and contact syncing, advanced collaboration tools.
  • Integrated spam protection and prevention against viruses and data loss.
  • Scalability makes it suitable for large and small businesses alike.

MailEnable. A Windows-based mail server that provides MTA, IMAP, POP3, and more. Its main features include:

  • Webmail interface, antivirus/spam protection, integration with Microsoft tools.
  • Easy setup and adjustable settings.
  • Hosting of multiple domains on a single server.
  • A good choice for small and medium enterprises.

PowerMTA. This Unix-based solution is aimed at companies performing large-scale email campaigns. Its distinguishable features are:

  • Excellent delivery speed, support for very large volumes, volume based licensing.
  • May be deployed on-premises or in a cloud.
  • Advanced real-time monitoring and reporting capabilities.

On-Premise MTA vs. Cloud SMTP Solution

Any of the two options might be best in particular cases. It all depends on your company's needs and resource availability. Before opting in for either one, we suggest comparing their features below. Also, do not miss our detailed comparison of an in-house server vs. an email service provider.  

 

 

On-Premise MTA

Cloud SMTP Solution

Infrastructure control

You maintain and own the hardware and software.

Offers maximum customization at the server level.

The provider handles maintenance and infrastructure.

Less server-level access or customization.

Setup and maintenance

All installation, fixing and updating stays within your organization.

Ongoing costs for hardware, power, and space.

Rapid setup, provisioned by the provider.

Less need for in-house technical resources.

Scalability

There is a need to purchase and configure the latest gear.

Can be time-consuming and costly.

Adjustments on demand.

Prepaid or subscription pricing plans.

Security

Direct control over security policies, software patches, and data handling.

Potentially higher security if well-managed in-house.

Provider employs enterprise-grade security measures.

Must trust vendor’s compliance with data regulations.

Deliverability support

IP warm-up, reputation, and feedback loops must be managed manually.

May need external tools or add-ons.

Often incorporates built-in IP warmup, reputation monitoring, and deliverability best practices.

Dedicated support teams.

Cost structure

Upfront hardware and software licenses.

Ongoing operating costs (IT staff, electricity, etc.)

Subscription or pay-per-use model.

Costs can be more predictable, particularly for steady or growing volumes.

Many organizations choose cloud-based or "managed" email delivery services like UniOne, which combine basic MTA functionality with lots of extra features. 

Try UniOne Free →

Now that you know what names to look for, let's look at the ideal MTA profile.

How to Pick the Best Mail Transfer Agent?

Choosing the right MTA means assembling vital components that directly influence email deliverability. It is the backbone for swift delivery, reliability, and overall business performance. Also, if you plan to send out a large volume of emails, it is indeed a critical component. So, to avoid messing it up, take the following steps.

Outline the Core Requirements

This step is pretty straightforward, yet sometimes underestimated. Define the volume of emails to go out. Will it be mostly marketing promos or transactional emails? The systems the team works on are also critical, as some mail transfer agents work on Linux only and some on Windows. Don't forget about the targeted industries’ rules. Are they regulated industries (finance, healthcare) requiring on-prem solutions, or are they all clear?

Research Available Options

Deep dive into others' achievements and what systems they use. See how the shortlisted solution can be applied in your case. Look for software with a history of reliability and strong community support.

Study Main Functions

Get to know all the functions and features of email MTAs and their correspondence to your business needs. Focus on the must-haves first. For instance, will it have built-in or third-party spam filters? Find out about queueing and delivery, reporting, and authentication protocols support.

Keep Security and Regulations in Focus

First, look at the data encryption to see which one is supported. Then, investigate the access control. Is it role-based administration, IP whitelisting, or two-factor authentication? Again, compliance, GDPR, HIPAA, and PCI DSS validate conformity readiness if operating in regions with strict data laws.

Evaluate Performance and Reliability

Look at performance benchmarks and user reviews. See how reliable your potential match is and how it handles large volumes of data.

Besides, capacity capabilities should not affect performance. If an MTA processes massive volumes but lacks stability, it's not that useful. Seek on-premise or cloud-based solutions that provide redundancy, such as failover servers.

Lastly, don't forget monitoring and analytics. Real-time logs and dashboards will give you critical insights into throughput, bounce rates, and delivery times.

Check Compatibility and Ease of Integration

Learn how future solutions will blend in with your existing email infrastructure. See how an MTA works with third-party apps and directory services. If there are any other essential tools or services within the company, it should be compatible with them, too. Look at what frameworks it supports. If you work with Node.js, Python, or PHP, check for existing integration libraries or wrappers. The better it blends with the current tech stack, the less time is needed for additional coding and workaround solutions.

Look Into Documentation and Help Resources

Bureaucracy is still part of the process, so careful evaluation of all documentation provided is a must. A comprehensive solution should provide extensive documentation, tutorials and dedicated support channels. Open-source platforms usually rely on online forums and community wikis, so check to see if these are active and useful.

Put It to the Test

It is always good to try before commiting. Spin up a test environment or a free trial. See how the solutions work when sending controlled volumes of emails for assessment purposes, discover throttling or queue bottlenecks. Analyse deliverability by evaluating bounce rates, spam folder placements, and email open rates.

Cost Considerations

Price is an important consideration when choosing a solution. Whether you have to buy a licence or use it for free, you’ll still pay additional operational costs. If you decide to go for on-prem solutions, be ready to purchase server hardware. For cloud-based options, you will need to pay for a subscription plan. Also, it is great to have to have some money on the side in case of any potential spikes.

Сonclusion

Now, there are no questions left about what a Mail Transfer Agent is. It is clear that its responsibilities cover almost everything email related, from filtering spam to authenticating messages. Whether you go for on-premise or cloud solutions, it can be the reason for the business' success or failure.

If you're looking to offload the complexities of MTA configuration and IP warm-up, cloud-based solutions like UniOne can handle them. Simply fill out the form and get a swift callback.

FAQ

Should I avoid using open-source MTAs?

Not necessarily. They are extremely reliable and widely used, and they also come with strong community support.  You may face a little struggle when it comes to dedicated resources to maintain and secure the system. An open-source MTA can provide maximum flexibility and transparency once you have the necessary skill set and infrastructure. If not, a cloud-based or managed service like UniOne might be better, as it can handle all the complexity.

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