Probably the most interesting addition is plug-in support. Previously, any extra features required patching the main source of the application, which generally resulted in messy and hard-to-maintain code, which was very tricky to upgrade. Now though, there is a complete plug-in API. Hopefully this will mean a veritable deluge of useful add-ons in the not-too-distant future.
For now though, I’ll happily make do with the default installation, as it already provides a handy selection of features. It may not be as versatile as SquirrelMail, but it is certainly better looking…!