The latest release is 3.13. This is an incremental upgrade with some new features, however old 3.0x configuration files should be fully compatible. Upgrading from previous releases is worth doing, particularly since a denial-of-service hole has been fixed over 3.0x and lower versions.
The previous release is 3.03. This was a major upgrade over the 2.1x series, and some features of the configuration file are not backward compatible - although there is a conversion utility. Earlier version 3.0x users should upgrade, although the bug fixes are relatively minor.
The last 2.x release version was 2.12. This is a bugfix release over earlier 2.1x releases. Upgrade from 2.0x releases is not imperative.
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.
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.