This is where binary search is not necessary. The admin could have just sent out unique messages, by making binary variations at 10 locations in the email (or t

This is where binary search is not necessary. The admin could have just sent out unique messages, by making binary variations at 10 locations in the email (or ternary variations at 7 locations, or quaternary variations at 5 locations, etc.). E.g. choose 10 words, that can be replaced with a synonym, then generate 2^10 = 1024 unique messages to identify the leaker.