I was about to say the same thing. For example if you go to someones GitHub profile and they have a public e-mail listed but you then go their repositories, filter by "source" (repos created by them, not forks) and you go to the oldest one they have of that kind, git clone that and then

     git shortlog -s -e

You might find that they were using their school e-mail back then for example.

Also if you clone all their repositories and everything else they have contributed to you might find an occasional commit in which they accidentally used for example the e-mail they have at work.