Whoever leaked that email is probably screwed.
A great technique to catch leakers is to encode into the email who received it. But all the text bodies are the same you might say!
Nah, you can easily insert a space into the email text in a few different places in some emails and not others, change the punctuation in some emails and not others, add extra capitalisation, even change words to words with similar meaning. You can narrow down the whole mailing list using such a code.
Then when the text is leaked you can match the leaked text back to the individual who was sent that version of the mail.