Every business need reliable email service. We have prepared a list of top on premise open source mail transfer agents for small and medium businesses.

Top Open Source Mail Transfer Agents

Linux has a monopoly on the internet when it comes to the software, it offers some of the best software for every business need. So, in the category of Transactional Email Linux offers many open source mail transfer agents and email solutions.

The Email has proved to be the fastest and reliable communication medium over the years. Therefore, from a large business to an individual, we all rely on email. So, due to the importance of email, it becomes very important to choose the right mail transfer agent which covers all your communication needs.

Here is a list of top 5 open source mail transfer agents that our team has prepared after going through tens of mail delivery software.


Postal is a full featured on premise alternative to Sendgrid and Mailgun. It comes with a simple HTTP API for easy integration with web apps. Postal is 100% free and open source, you can download the source code modify it as per your requirements.

Its notable features among others are as following:

  • Support for multiple organizations with mail servers & users within.
  • Graphs and stats showing volume of incoming & outgoing mail.
  • Access to view the full outgoing & incoming message queue.
  • Set up webhooks to receive live information about delivery information in realtime.
  • Built-in DNS checking & monitoring to ensure domains you send mail from are configured correctly for maximum deliverability.
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Cuttlefish is a self hosted Mail Delivery Software and is widely used to send large volumes of emails with ease. It supports the SMTP protocol. Consequently, allows you to send emails from your application with no effort. Cuttlefish uses postfix for email delivery which is one of the most popular and reliable open source mail transfer agent. Therefore, it has all the reliability of Postfix.

Here are the notable features:

  • Ensures your emails arrive at their destinations.
  • Keeps your email reputation high by automatically holding back emails to destinations that have previously bounced.
  • Monitors in real time which emails arrive at their destination and which bounce.
  • Sends email from your application using smtp in the usual way and get all sorts of added benefits for no effort.
  • Works with any web framework and language
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Mail-in-a-box is an innovative Linux mail server that focuses on easy deployment. It provides handy webmail and IMAP/SMTP server with automatic DNS configurations. It helps you to setup your own Gmail with no technical expertise. Its top features include:

  • Automatic DNS configuration
  • Spam filtering.
  • Greylisting.
  • Backups to Amazon S3, static website hosting.
  • Free TLS (SSL) certificates from Lets Encrypt.
  • Host mail for multiple users and multiple domain names.
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Postfix is a powerful open source mail server that can be used for routing and delivering emails. However, It was originally developed to replace the popular mail transfer agent Sendmail. Even though it follows Sendmail a lot. But, in the implementation it is quite distinct and unique. It offers many features including

  • Junk mail control
  • Supports multiple protocols
  • Database support
  • Mailbox support
  • Address manipulation support and many more
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Exim is a full featured mail server used on many Unix-like operating systems. It offers cross platform support which makes it a good choice for mail servers. Exim is a highly customizable server for email routing and delivery. Exim is like a framework with its application language to setup complex configurations. It has an advanced multi-step mail processing logic which helps it to solve complex use cases. Here are the top features of Exim:

  • Supports protocols such as RFC 2821 SMTP and RFC 2033 LMTP email message transport
  • Configurations include access control lists, content scanning, encryption, routing controls among others
  • Excellent documentation
  • It has utilities such as Lemonade which is an assortment of SMTP and IMAP extensions to enable mobile messaging plus many more.
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In this article you have reviewed the best open source mail transfer agent. Firstly, we listed the main highlights of each short listed MTA. After that, we explained how these features make them unique. In conclusion, if you are looking for a quick and easy solution without complex configurations then Postal, Cuttlefish and Mail-in-a-box are your choices. However, if you are looking for more advanced control via configurations then you can choose Exim or Postfix.