What Is a Mail Transfer Agent?

email deliverability checklist cover
The confusing part of this topic is that you might have already heard of this type of software before.

A Mail Transfer Agent (MTA) has many acronyms. Some of them are:

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

Even if these terms are completely new to you, the function is simple and crucial.

An MTAs are highly important to avoid any deliverability issues.

The following article will give you the necessary info about MTAs and will present some of the most used solutions.

Let’s dive right into it.

What is an MTA (Mail Transfer Agent)

A Mail Transfer Agent is a piece of software that relays emails between computers using SMTP(Simple Mail Transfer Protocol).

SMTP is an email protocol used for sending email messages from one email account to another via the internet.

In essence, an MTA serves as an email traffic management system to make sure that your email travels from point A (the sending client) to point B. (the receiving client).

An MTA is responsible for the core tasks involved with delivering email, including queuing, throttling, scheduling, connection management, data transfer, processing of deferrals, bounce generation, and tracking of the delivery status.
All of the operations that MTAs do utilize the Simple Mail Transfer Protocol (SMTP). MTAs perform the tasks outlined by the protocols, and SMTP works to transfer data as intended.

Does MTA Affect Email Deliverability?

Yes, your email deliverability is affected by an MTA.

Email deliverability is based on factors like sender reputation, authentication, and engagement.

The most crucial factor is the reputation of the IP address the email is sent from. All emails from that sender will land in the spam folder or possibly bounce back when receiving mail servers flag the sender as being unreliable. MTAs can uphold and improve the sender's reputation. They, therefore, have a direct bearing on email deliverability.

Reputation management is the number one factor in deliverability. From building email reputation to warming up new IPs, or understanding send limits per domain to DKIM signing, the MTA is crucial in assisting in optimum deliverability. The right MTA allows email marketers to define the deliverability rules to suit the sender and the ISP.

Warming up new IP domains

New IP addresses lack any sending reputation and are useless for sending email campaigns.
The quantity of connections or messages that receiving domains accept is typically limited.

There are many options that an MTA can use to warm up IP addresses and raise transmission limitations. For example, an MTA can gradually expand your sending capacity without damaging your reputation.

Another strategy is to use cold IP addresses in addition to your usual transmitting IPs and route communications there with very low limitations. You ought to be able to adhere to those limits, thanks to your MTA.

Using routing features like delivery throttling, flood patterns, or dynamic delivery can be accomplished.

Signed DKIM

A signed message makes it easier for ISPs to recognize reliable domains. This feature and your IP reputation are comparable. ISPs can verify messages with a DKIM signature. In other words, ISPs can verify that a message came from a particular domain.

Based on the reputation of that domain, it can decide whether to put a message in the inbox. In general, signed messages have a higher likelihood of being authentic.


The reputation of the sender frequently suffers when they are added to a blacklist. Some email providers employ greylisting as a kind of preventive action. A valid sender can bypass this filter far more easily than they can the blacklist. In order to accomplish this, an MTA sets up many queues and makes numerous tries to deliver emails that have bounced.

MTAs give you a lot more options in addition to the previously mentioned ones. They can be used for many different things, such as creating email throttling and routing rules, keeping track of the flow of outgoing mail, and more.

Best Mail Transfer Agents


One of the first and most well-known MTAs for Linux server platforms, Sendmail (also known as Proofpoint), employs TCP or IP networks and SMTP protocols to send and receive messages. It only sends emails when installed on an email server.

It is portable, completely customizable, and a dependable source that administrators may control from a distance. It has several benefits, but it also has certain drawbacks. The configuration procedure is difficult, it doesn't have Post Office Protocol (POP) or Internet Message Access Protocol, and it provides shoddy security methods (IMAP).

  • The lightweight and fully configurable MTA is suitable for large and complex environments.
  • Sendmail is a highly portable solution.
  • Flexible and easy to use.
  • Makes use of configuration files for controlling and managing mail configuration.
  • Allows creating own mailing lists.
  • Offers failure messages to a client if an error is found in transferring messages to the recipient.
  • Requires minimum server resources to run the software.


A free, open-source, cross-platform server called Postfix was initially created as a rival to Sendmail MTA. This cross-platform was created and developed by Wietse Zweitze Venema while he was working on a project at the IBM research division.

The goal was to create software that could address Sendmail's shortcomings while assisting with route-finding and email-sending.

It provides straightforward configuration, secure operation, simple administration, and flexibility in working. Additionally, it performs quickly and is compatible with Mac OS X, Linux computers, and other Unix-like operating systems.

  • Junk mail control.
  • Offers Database and Mailbox support.
  • Supports numerous protocols.
  • Address manipulations and offers multiple layers of defense.
  • Postfix is a fast, flexible, and robust MTA.
  • Uses table lookups.


It is a safe, open-source mail transfer agent (MTA) that is free to use and can be installed on Unix-like operating systems. With advanced security measures, it serves as a different MTA for Sendmail. More quickly than Sendmail, it is a fully modular system that can manage mass mail operations.

Additionally, it is a straightforward, dependable, and effective MTA that fully supports VERPs and address groups. The software offers automatic host configuration as well.

  • Qmail is free, open-source MTA, and is quick to install.
  • Allows users to create and manage their own mailing list easily.
  • Supports address groups.
  • Offers automatic prevention of mailing list loops.
  • It forestalls mailing list circles.


Cuttlefish's outstanding web interface makes it a top pick. Users find it simpler to use and monitor the program, thanks to Cuttlefish. It enables administrators to examine and manage IP reputation quickly.

Cuttlefish supports the SMTP protocol and makes it simple to keep an eye on all of your incoming and outgoing emails. Additionally, you may keep track of any communications that succeeded or failed and bounced back. Additionally, you can see every email that has been opened by users, as well as the links that they have visited.

Despite not supporting HTTP API, it is among the best pieces of software. Additionally, postfix is used for email delivery and routing.

  • The web interface helps in easy monitoring.
  • Supports SMTP protocol.
  • Real-time analysis.
  • Supports email tracking.
  • Allows checking IP reputation.
  • A reliable and flexible mail server that ensures delivery to the destination.

Cloud Infrastructure Services list these and many other MTA solutions. Check out their blog for more info.

Increase Deliverability with Warm

Especially for cold mailing and email marketing, deliverability plays a major role. However, an MTA is not always needed. As most companies use Gmail to send emails, it’s common to make use of Google’s SMTP and MTA infrastructure.

The only thing that is missing is a proper way to warm up your inbox, monitor your overall sending efforts, and check your deliverability for sending cold emails or email newsletters.

With Warm, you make use of the automated campaign warm up.

Campaigns will be automatically sent from your mailbox, and the tool will positively interact with your emails. As a result, your deliverability is boosted, and your email address is ready for sending emails at scale.


Having a Mail Transfer Agent is a great option to increase deliverability and, at the same time, increase the security of your email traffic.

Finding the right MTA that fulfills your needs is crucial not to lack any security issues. However, for most companies, Google’s MTA and SMTP infrastructure is sufficient in combination with a deliverability checker like Warm.

Get in touch with us to find out if Warm is the tool you’re looking for.
Get notified when we're live!