Send a mass email from a spreadsheet

One more lesson learned from my first Kickstarter.

Once The Webcomics Handbook was finished, I was faced with the perplexing problem of how to deliver a PDF file to 1,016 backers. Sending individual e-mails was out of the question. Using Dropbox was an option, but since I was going to begin selling this book digitally, I wanted to keep everything in one place. I was also  concerned about the ability to distribute updates to the book as the need arose.

So how does one send over a thousand e-mails distributing separate download codes among specific e-mail addresses?