1.epub: free to use,
2.mobi: from epub ,
3.azw and azw3: more styles fonts layouts; for calibre and alfa;
4.iba: apple's ibboks author app;
Send to Kindle can convert and deliver the following types of documents:
EPUB (.epub)
Adobe PDF (.pdf)
Microsoft Word (.doc, .docx)
Rich Text Format (.rtf)
HTML (.htm, .html)
Text (.txt) documents
Compressed documents (.zip, .xzip)
JPEG (.jpg), GIF (.gif), Bitmap (.bmp), and PNG (.png) images.
If the format of the document that failed is supported (listed above), please ensure the document is not password protected or encrypted, and try sending it again.
Other helpful tips for sending documents to Kindle via email:
Limit attachments to fewer than 25 documents per email.
Individual file size should be less than 50 MB (before compression in a ZIP file).