Have you had tried to send a file to your kindle in the past 6 hours or so? If you succeeded, congratulations. There's a rash of problem reports right now about the automated email service provided by Amazon. It's not working.
I don't know why it's not working but I know for damn sure it's broken. I've got a dozen or more reports from frustrated users (here, here, here, and even a tweet from Readability). I know a number of them are experienced Kindle owners, so it's quite obvious that the problem isn't with them.
Amazon's servers are broken somehow. But I wouldn't worry too much. They're probably fixing it right now.
Update: Amazon are working on it and expect to have it fixed in a couple hours (~2 am eastern).
image by miss_rogue