Calibre is for a number of operating systems:
http://calibre-ebook.com/download You can email files from Calibre directly to your Kindle, too (I do that a lot).
How are you sending the mobi files to your Kindle (emailing them?) Are you sure the sender / site is on your "approved" sender list? You have to allow which email addresses can send documents to your Kindle, so make sure they're on your allowed list under Manage Kindle on the Amazon site.