I have a bunch of EPUB formatted books. What I have previously been doing is converting to AZW3 with Calibre and transferring them directly to my Kindle.
What I would rather do use the "email to Kindle" feature so I can them have them stored in my Amazon account and synchronize reading position etc between devices.
The problem is that Amazon reject AZW3 files and MOBI files using the "new" format when emailed to a device. They seem to only accept the "old" MOBI format.
So hear comes the question: am I losing anything by converting these books to the "old" MOBI format rather than AZW3?