Currently, in the Agent Interface, admins can display the "User list".
In this list, the registered e-mail may be empty.
I guess it concerns people who avoid creating a UserEcho and prefer to user third-party login (like Google, Facebook, etc.)
Wouldn't it be possible, in that case, to replace the missing e-mail by the source of authentication (Google, Facebook, ...) or even the username of that third-party system?
That would be an interesting information to observe what login people mainly use.