All described below is only in affect, when user has not specified email during registration or has removed it later from "My Profile" section.
1) When user have completed registration process hi/she receives the message: Thank you for registering on our website. Your registration is pending administrative approval. You will get a separate email once it's activated.
The user, was registered without e-mail, and will not get any e-mail in any case.
2) Forgot password functionality can't possibly work for users, who have not specified email in their profiles. So if user without email tries to email new password to himself, then we should display appropriate message, like "You have not specified email in your profile and can't use forgot password functionality".
3) When user enters his username into forgot password form, then it won't be found, because of missing email address in his profile. Really strange.