NOTE: Amazon has updated their terms of service for reviews on Oct 3rd 2016.
They honor giving out review copies of your book, but not as an incentive for a positive review. It's a little vague and confusing, but they want to avoid people getting free stuff for positive reviews.
You may give out review copies of your book, but no longer in direct exchange for a positive review.
Readers that choose to review something can do so on their own. Without your prompting or influence.
Amazon is being vague so play it safe and use Amazon's "wording" directly when soliciting for a review. Example:
Attached is a review copy of my book. It's the first in a fantasy series about two warring elf nations. Hope you like it!
Notice I kept it short, and didn't even request a review. Provide your book as is. Let the reader decide what to do with it.
NOTE: Never offer them money or any type of incentive. NEVER EVER. They will block you or worse report you to Amazon. Amazon allows you to give out review copies of your book. Always check with Amazon for the latest in review policy (it changes frequently).
Also NOTE: Depending on your email program you may need to remove the name before the actual email manually.
These are publicly available email addresses openly looking for review solicitations. These email address are listed on forums, Amazon account profiles, review blogger sites, etc.
Also these guys all review books, regardless of their name (like "productReviewguy"). Email them all!