Unfortunately Nabble does not provide it. The user has to provide his e-mail upon registration so he should know his e-mail is going to be shared with the system.
You can tell on your e-mail that it is being sent by Nabble forum and his e-mail was not made available to you.
Try to send an e-mail to one of your users and you will notice his e-mail does not show up on the form.
Nabble staff. We never ask for passwords.