docs: def clean(self, *args, **kwargs):
          """
          Custom validation
          """
          if self.is_scheduled is 1 and (self.scheduled_date == '' or self.scheduled_date is None
                                         or self.scheduled_time == '' or self.scheduled_time is None):
              raise ValidationError(_('If message is scheduled both fields "scheduled date" and "scheduled time" must be specified'))
  
          if self.is_scheduled is 1 and self.scheduled_date < now().date():
              raise ValidationError(_('The scheduled date is set to a past date'))
  
          if self.is_filtered is 1 and (len(self.filters) < 1 or self.filters == [''] or
                                        self.filters == [u''] or self.filters == '' or self.filters is None):
              raise ValidationError(_('If "recipient filtering" is active one of the filtering options should be selected'))
  
          if self.is_filtered is 1 and FILTERS.get('groups') in self.filters and\
             (len(self.groups) < 1 or self.groups == [''] or self.groups == [u''] or self.groups == '' or self.groups is None):
              raise ValidationError(_('If group filtering is active at least one group of users should be selected'))