select id, email,country_id, rown, SUBSTRING(email, INSTR(email,'@') + 1) as domain from 
(
select ta.*, SUBSTRING(email, INSTR(email,'@') + 1) as dom,
if(@typex=SUBSTRING(email, INSTR(email,'@') + 1), 
@rownum:=@rownum+1, 
@rownum:=1+least(0,@typex:=SUBSTRING(email, INSTR(email,'@') + 1))) rown 
from xx_users ta, (select @rownum:=1, @typex:='_') zz
order by dom, registred DESC
) yy
where rown < 6

CREATE TABLE `xx_users` (
  `id` int(64) NOT NULL AUTO_INCREMENT,
  `email` varchar(120) CHARACTER SET latin1 NOT NULL DEFAULT '',
  `name` varchar(50) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
  `surname` varchar(50) COLLATE utf8_unicode_ci NOT NULL DEFAULT '',
  `date_of_birth` date NOT NULL DEFAULT '0000-00-00',
  .
  .
  .
  .
  PRIMARY KEY (`id`),
  UNIQUE KEY `email` (`email`)
) ENGINE=MyISAM AUTO_INCREMENT=609581 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;