The latest release is 4.05. This is a major upgrade over the 3.x versions which will require configuration files to be rewritten (there are tools to help with this). This release is a recommended bug fix update over earlier 4.0x versions.
The latest 3.x release is 3.36. This is the latest bug fix release in the 3.3x series. They are an incremental upgrade over the 3.2x series with a few new features. There is a security issue (rare but exploitable) with versions previous to 3.34 so upgrading is strongly recommended. Old 3.0x configuration files should be fully compatible. New features are documented in the CHANGES and NewStuff files.
The previous 3.2x release is 3.22. This is an incremental upgrade with a small number of new features and some bug fixes, however old 3.0x configuration files should be fully compatible. New features are documented in the CHANGES and NewStuff files Upgrading from previous releases is worth doing, particularly since a denial-of-service hole has been fixed over 3.0x and lower versions.
There were a number of early beta versions of exim with version numbers less than 1.0. These should now be avoided. Versions earlier than 2.0x should also be upgraded. Version 3.0x and 3.1x releases are also best upgraded.
There may be beta versions with available from the ftp sites in the Testing directory. Many people are using these without problems, but they are not recommended unless you are willing to work with beta software.