TABLE mailmsg (
	id BIGINT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
	id_from INT UNSIGNED NOT NULL REFERENCES mailaddr(id),
	body MEDIUMTEXT,
	INDEX (id_from)
	);

TABLE mailaddr (
	id INT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
	addr VARCHAR( 64 ),
	FULLTEXT (addr)
	);

TABLE mailto (
	id BIGINT UNSIGNED NOT NULL REFERENCES mailmsg(id),
	id_to INT UNSIGNED NOT NULL REFERENCES mailaddr(id),
	INDEX (id_to),
	INDEX (id)
	);

mysql> select count(*) from mailmsg
    ->  inner join mailaddr on (mailmsg.id_from=mailaddr.id)
    ->  inner join mailto on (mailto.id=mailmsg.id)
    ->  where mailaddr.addr='xxx@some.ru';

+ ----------+
 
| count(*) |
+ ----------+
 
|         2  |
+ ----------+
 
 1  row in set ( 17 . 67  sec)

| table    | type   | possible_keys   | key     | key_len | ref             | rows  | Extra       |
+ ----------+--------+-----------------+---------+---------+-----------------+-------+-------------+
 
| mailto   | index  | id              | id      |        8  | NULL            |  51980  | Using index |
| mailmsg  | eq_ref | PRIMARY,id_from | PRIMARY |        8  | mailto.id       |      1  |             |
| mailaddr | eq_ref | PRIMARY,addr    | PRIMARY |        4  | mailmsg.id_from |      1  | where used  |

select count(mailmsg.id) from mailmsg inner join mailto on mailmsg.id=mailto.id where  (mailmsg.id= 1  or id_to= 1 );

+ -------------------+
 
| count(mailmsg.id) |
+ -------------------+
 
|                190  |
+ -------------------+
 
 1  row in set ( 0 . 32  sec)

+ ---------+--------+---------------+---------+---------+-----------+-------+-------------------------+
 
| table   | type   | possible_keys | key     | key_len | ref       | rows  | Extra                   |
+ ---------+--------+---------------+---------+---------+-----------+-------+-------------------------+
 
| mailto  | ALL    | id,id_to      | NULL    |    NULL | NULL      |  51981  |                         |
| mailmsg | eq_ref | PRIMARY       | PRIMARY |        8  | mailto.id |      1  | where used; Using index |
+---------+--------+---------------+---------+---------+-----------+-------+-------------------------+