docs: def clean(self, *args, **kwargs):
          """ custom validation """
          if not self.user and (not self.from_name or not self.from_email):
              raise ValidationError(_('If sender is not specified from_name and from_email must be filled in.'))
          # fill name and email
          if self.user:
              self.from_name = self.user.get_full_name()
              self.from_email = self.user.email