'This is a literal copy from Django 1.4.5\'s django.contrib.auth.forms.PasswordResetForm
Except removing the requirement of active users
Validates that a user exists with the given email address.'

def clean_email(self):

    email = self.cleaned_data['email']
    self.users_cache = User.objects.filter(email__iexact=email)
    if (not len(self.users_cache)):
        raise forms.ValidationError(self.error_messages['unknown'])
    if any((user.password.startswith(UNUSABLE_PASSWORD_PREFIX) for user in self.users_cache)):
        raise forms.ValidationError(self.error_messages['unusable'])
    return email
