I understand not sending the reply via actual email, but the reply should be visible if you want to make this fair + an actual iterative learning experiment.
To prove my point, c5633e6781ede1aea59db6f76f82a365 is the md5sum of an email address. What's the email address?
If the attacker already knows a given input email ('foo@gmail.com'), then any hash algorithm will identically let them see the emails.
The problem with the above proposal isn't related to hashing, it's that the email address is being used as a password to see sent contents, which seems wrong since email addresses are effectively public.
