An MTA routes and delivers email messages.

Learn the importance of MTAs in Internet email handling systems, how they work, and factors to consider when selecting a solution. Explore more here.

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Emails transfer happens between the computers of a sender and a recipient using software called a mail/message transfer agent (MTA). Exim, Postfix, Sendmail, Qmail, and Microsoft Exchange are some common MTAs.

Mail relay, mail router, and Internet mailer are all names for a mail server. But an MTA is the most specific one. MTAs are crucial in Internet email handling systems since it exchanges user-to-user electronic mail messages.

How does MTA work?

An email is sent to MUA(Mail User Agent) or the email client, then travels to the MSA(Mail Submission Agent). Further, it is sent to MTAs where the relaying takes place. Afterward, the email drops into the recipient's Mail Delivery Agent (MDA) and finally to the MUA destination.

If the receiver is not a local hosting, the email is sent to other MTAs. POP3 or IMAP4 are utilized at the end. MTAs query the MX records to select a mail server. They send auto-response messages if an email fails to reach the destination.

Mail queueing in MTAs

MTAs handle mail using a store-and-forward model. First, the mail is placed in a queue and awaits a response from the recipient's server. Then, if not delivered within the specified terms, the mail returns to the mail client.

Does MTA impact email deliverability?

Email deliverability is affected by the infrastructure, authentication, and content. For example, if we identify the sender as unreliable, all the emails end up in the spam folder or bounce back.

MTAs protect and strengthen the reputation of the sender, impacting email deliverability.

What does a mail transfer agent do to build your sending credibility?

If you create your reputation from scratch, you shouldn't utilize a virgin IP address at full load. This is because it has no email history and thus needs warming up. Each receiving domain sets its incoming mail limits. If exceeded, the sending mail server is identified as untrustworthy. To avoid this, configure your MTA to limit sending. Greylisting is a filter a sender can get as a preventive measure easier than the blacklist.

Factors for choosing an MTA solution

Some MTAs are open-source, while others require payment.

When choosing an MTS solution, consider:

  • MTA’s performance – speed, volumes, and latency of sending emails.
  • MTA’s configurability – entails access to specific configurations improving the performance

Before choosing an MTA vendor, evaluate:

  • reputation and credibility of the vendor
  • pricing, infrastructure transparency
  • security policy, data protection mechanisms
  • user support for troubleshooting

Factors improving MTA performance:

  • throttling, IP pools
  • email authentication, monitoring (SPF, DKIM, DMARC)
  • multiple queuing
  • spam control

On-premise MTA vs. cloud-based SMTP relay – which is better?

Each project has specific needs and can benefit from a home-based or cloud-based solution.

An on-premise MTA

Enterprises and big companies use on-premise mail transfer agents. This entails a full-fledged email infrastructure set up according to your requirements.


  • Manages every email sending configurations
  • Improved dependability
  • Can send tons of emails without delays


  • Are expensive, and setting up is time-consuming
  • Need space to accommodate hardware
  • Hard to scale

A cloud-based SMTP relay service

For moderate email sending needs, we use a cloud-based infrastructure. This is because you share the infrastructure with others and don't have complete control. However, some businesses offer dedicated IPs as an optional feature.


  • Easy to set up and use
  • Flexible pricing
  • Easy to scale


  • Costly in the long-term perspective
  • It depends on the SMTP relay service
  • Lack of control

All the advantages of the on-premises option are accessible to those with money—those who don't will experience some limitations but will still have options.