Exim version 3 was a popular MTA used by many sites and the standard MTA used on some Operating System distributions - most notably Debian GNU/Linux.

Exim version 4 was released in February 2002. Since that time there has been one final release of Exim version 3 - version 3.36, but no further releases or development have happened and none are planned.

Exim 3 is no longer being maintained by the Exim developers.

It is believed that there may be security issues in Exim 3 (specifically some of the Exim 4 security issues may also affect Exim 3), however the Exim developers are not issuing updates against Exim 3. For this reason if you wish to use Exim 3 then you should use a distribution version where the distributor is looking after security updates.

The major system for Exim support is the exim-users mailing list - see EximMailingLists. However none of the main posters on the list (those who would normally answer questions) use Exim 3 any more - all have upgraded to Exim 4. The differences between the two versions are such that it is very difficult to give accurate information for Exim 3 any longer.

The documentation for Exim 3 is still online from the main website.

All users should update to Exim version 4. Most Operating System distributions have been using Exim 4 for some time.

Exim version 3 was the default MTA in Woody, the previous version of Debian stable. It is still available in Sarge (the current stable release), packaged as exim. Exim 4 is available packaged as exim4, which is the default MTA for new installs. Systems upgraded from Woody to Sarge may still be running Exim 3.

While the non-Debian exim community still strongly recommends that users upgrade to Exim version 4, Debian will continue to support exim 3 for as long as it remains in the Debian GNU/Linux distribution. There will be exim 3 packages available for some time. Debian is committed to deliver security fixes for exim 3 within the scope of a volunteer-driven Linux distribution. However, it is important to note that current versions of Exim have many more eyes actively looking for security flaws, and writing fixes.

As exim4 is an entirely different package from the exim package which contains exim3, and exim3 and exim4's configuration are quite different, there is unfortunately no seamless upgrade process. On installation, the exim4 package tries to guess from an exim 3 configuration file which might be found on the system which answers were given during exim 3 configuration and seeds its configuration questions appropriately. However, there will be questions asked during installation. This automatic parsing process might fail if you did elaborate things with your exim 3 config.

Although you might be able to use the convert4r4 script to convert your exim 3 config, the Debian maintainers recommend that you use its output only as a basis for your local modifications to the exim4 config that came with the package. That way, you will still see their changes when we do changes to the default configuration which might let you profit as well.

More information about Debian's exim4 packages is available on DebianExim4.