This puzzle. It kinda ended with a fizzle, which is a bit sad.
Basically, if you view the e-mail’s source (most e-mail clients have a “view raw message” button or similar hidden somewhere), there’s two commented-out lines hidden - each of the sixty level-up emails has a unique pair of lines. Putting the line pairs in the right order (i.e. such that the first letter of the first line of each pair matches the first letter of the second line of the previous pair) yields a story.
The real trick is that reading every odd-numbered initial letter yields a phrase - running every other letter through a Vigenere cipher using the first phrase as the decoder key yielded a question, the answer of which was the password for that puzzle.