[ryan-c]

The search space of email is small enough that hashing is not a good solution.

http://www.developer.it/post/gravatars-why-publishing-your-e... http://archive.hack.lu/2013/dbongard_hacklu_2013.pdf

[richardwhiuk]

They can add a random seed to the hash to increase the search space. In fact it doesn't need to be a hash - they could just store a random string.