I had sent someone a message via the contact user form, explaining that I was unsetting their email address and they would need to re-add and reconfirm it.
In this case it happened to be a valid email address, and the user sent me a message via the contact user form asking how they can re-add their email address... but I can't reply to them, because sending a message requires that the target user has an email address set. >_<
I would expect people to only be able to send a message if they can be contacted themselves, otherwise people can't reply to them. If they don't have a validated email address, it should display a message and link to the page where they can do that.
(Curious what happens if you try to reveal your email address when you have none set...)