Disable email registration requirement in Drupal 7 I've already removed e-mail validation, but the requirement that each user supply a unique e-mail address needs to be removed too. How can I go about doing that without resorting to a messy workaround like dummy e-mail addresses (which cause problems like making contact forms silently fail)?
For removing unique e-mail address requirement see Shared Email module:


  The shared email module allows multiple users to use the same email address for different accounts. The module works for both registration and account updates.


Disabling email requirement for registration is quite complicated task because many things in Drupal use mail notifications. I can recommend to set special email users.without.mail@yoursite.com for users without emails.