See:
http://www.amazon.com/sendtokindle/email where Amazon lists the accepted input formats. Note that MOBI is supported but not AZW3 (because Amazon is apparently behind the eight ball in making use of their own in-house format

) although you can send a dual-MOBI (kindlegen makes these by default, calibre can make them if you set the right Conversion ==> MOBI Output options).
If you send dual-MOBI, then the Personal Documents service will split it into AZW3 for supported devices but not show a cover. MOBI will work properly.
Also, welcome aboard!