This being a bug-fix release, there have been no exciting new features implemented since 2.2.1 -- just heaps of fixes.
With special dispensation from the "bugfixes only" rule, a brand new version of the email package (nee mimelib) is also included: email 2.4.3.
For a partial list of these fixes, please see the release notes, or the Misc/NEWS file in the source distribution. For the full list of changes, you can poke around CVS.
>>> release notes