Amazon’s Kindles are some of the best dedicated e-readers you can buy, but a longstanding criticism from users of DRM-free books from non-Amazon sources is that they don’t support the open ePub standard. That has changed at some point in the recent past, as spotted by Good E-Reader: Amazon’s Kindle Personal Documents Service will now accept ePub files sent to your device’s Send to Kindle email address, the same way it currently handles PDF files, Word documents, and other image and text files.

Source: Kindle e-readers finally (kind of) support ePub books

Finally. I guess they use ePub internally a lot too.