[reeses]

You're underestimating the level of effort here. It's not about lacking the $100 (they've received more than that in donations), it's that Apple makes significant changes to the way their applications are built and run and they don't communicate the changes very well, if at all.

In addition, as the apps are linked and compiled with different settings (clang, no ppc, no 32-bit, etc.) then plugins that are dependent on these APIs have to find their own way.

ML was a huge change from Lion in many ways. Mavericks has some issues that an individual can resolve on their own (mostly) but it's a moving target as it is not released yet.

But I do encourage you to donate. Integration with your native mail app should be worth $50-100+.