I have a Kindle Fire, so I'm not sure if this is necessary for all Kindles but on my Amazon account page is says "To prevent spam, your Kindle will only receive files from e-mail addresses you have authorized."
To authorize e-mail addresses:
Go to Amazon.com --> Manage Your Kindle. On the left side of the page, click on Personal Document Settings then, under the heading Approved Personal Document E-mail List, click on Add a new approved e-mail address.
