A Kindlet would indeed be awesome. Still... (and I hate to say this): you're quite probably underestimating the complexity of the task.
First, there is no support for the standard Swing classes (JButton, etc.) on the Kindle 3. Instead, Amazon chose to create their own versions (KButton etc.), but support is far from being complete. So for instance, you even have to reinvent the wheel to simply have a checkbox or radiobox component. Developing a non-trivial UI for the K3 is really a PITA, and it still looks ugly afterwards
This has changed with the Kindle 5, where you can simply use the standard Swing components - that makes developing a Kindle app pretty much the same as developing a normal UI application. (I'm not sure which version the K4 supports, I don't have one). The bottom line is that it seems to be pretty much impossible to code an application which can work on all Kindle models.
Second, you'll need some functionality not normally accessible to Kindlets, like network access, and probably filesystem access. This could be solved by using my Kindlet Jailbreak (yes, it does work on the K3).
And third, of course, a mail client is not a trivial piece of software to code (though as you said, there may be third-party libraries available, which could at least help with all the lowlevel stuff).
Still: good luck!
