Simple.

Click on 'My Details' above.

Then click on 'Edit Profile'

Scroll down to the bottom of that box and you'll see a question that asks 'email visible to other users' - click the no box and then hit save.

That's it, now people won't be able to see your email address, but they will be able to leave private messages for you.

Once you start to trust the person who is sending private messages, you can exchange proper email addresses if you wish.