Exim 4.95 MTA Comes with a Bunch of Improvements

The latest version of the popular mail transfer agent, Exim 4.95, ships with TLS resumption support included in default builds.

If you’re unfamiliar with Exim, it is one of the most used email server software, developed at the University of Cambridge. It is a free software under the GNU General Public License terms, which means that Exim is an open source mail transfer agent (MTA). It is among the best mail transfer agents in regard to its customization and configuration abilities.

Exim runs primarily on Linux or Unix and is the default MTA on Debian, though Ubuntu and Red Hat Enterprise Linux use Postfix by default.

An interesting fact to note is that Exim is currently the world’s most popular MTA. According to one recent survey nearly 60% of mail servers visible on the internet use Exim, followed by Postfix at 35%. 

Exim Market Share

Recently, the project has released Exim 4.95, so let’s take a quick look at it.

Key Changes in Exim 4.95

The native SRS support, previously experimental, is now supported. In short terms, SRS (Sender Rewriting Scheme) does exactly what its name says – it’s a scheme for rewriting an email’s envelope sender in order to remail it.

Perhaps the most important new feature introduced in Exim 4.95 is a TLS session resumption, which is now supported and included in default builds. As the name suggest, TLS session resumption provides a mechanism to resume or share the same negotiated secret key data between multiple connections. It provides two important benefits:

  • Eliminates an extra handshake roundtrip for returning visitors, which results in speed improvement.
  • Reduces the computational cost of the handshake by allowing reuse of previously negotiated session parameters.

The other big change in this update concerns the faster TLS startup. When various configuration options contain no expandable elements, the information can be preloaded and cached rather than the previous behavior of always loading at startup time for every connection. This helps particularly for the CA bundle.

In Exim 4.95, the default maximum number of recipients of a single email was changed from “unlimited” (ie: as much as CPU and memory will allow, until something breaks badly) to 50,000. Of course, you can raise or lower this as you see fit, but Exim’s devs strongly caution against using zero/unlimited.

Among other noteworthy changes, Exim 4.95 adds an option to ignore the cache on a lookup and support for single-key LMDB lookups. In addition, the fast-ramp two phase queue run support, previously experimental, is now supported by default.

Note that there may be some incompatibilities if you upgrade from an older version to the latest release. The names of various drivers such as authenticators, routers, transports, etc, have always been limited to 64 characters, but before Exim 4.95 the names were silently truncated, inviting problems. Now the length limit should be enforced. If this affects you, then please rename to use shorter names.

For more information about all changes in the new version, you can refer to the full changelog or you can visit the project’s website.

Bobby Borisov

Bobby Borisov

Bobby, an editor-in-chief at Linuxiac, is a Linux professional with over 20 years of experience. With a strong focus on Linux and open-source software, he has worked as a Senior Linux System Administrator, Software Developer, and DevOps Engineer for small and large multinational companies.

Think You're an Ubuntu Expert? Let's Find Out!

Put your knowledge to the test in our lightning-fast Ubuntu quiz!
Ten questions to challenge yourself to see if you're a Linux legend or just a penguin in the making.

1 / 10

Ubuntu is an ancient African word that means:

2 / 10

Who is the Ubuntu's founder?

3 / 10

What year was the first official Ubuntu release?

4 / 10

What does the Ubuntu logo symbolize?

5 / 10

What package format does Ubuntu use for installing software?

6 / 10

When are Ubuntu's LTS versions released?

7 / 10

What is Unity?

8 / 10

What are Ubuntu versions named after?

9 / 10

What's Ubuntu Core?

10 / 10

Which Ubuntu version is Snap introduced?

The average score is 68%