mysql> EXPLAIN SELECT * FROM cupid.cupid c LEFT JOIN cupid.cupid_msgs cm ON (cm.receiver_id = c.member_id) WHERE cm.receiver_id IS NULL;
+----+-------------+-------+------+---------------+---------+---------+-------------------+---------+--------------------------------------+
| id | select_type | table | type | possible_keys | key     | key_len | ref               | rows    | Extra                                |
+----+-------------+-------+------+---------------+---------+---------+-------------------+---------+--------------------------------------+
|  1 | SIMPLE      | c     | ALL  | NULL          | NULL    | NULL    | NULL              | 1840658 |                                      |
|  1 | SIMPLE      | cm    | ref  | PRIMARY       | PRIMARY | 4       | cupid.c.member_id |  226603 | Using where; Using index; Not exists |
+----+-------------+-------+------+---------------+---------+---------+-------------------+---------+--------------------------------------+
2 rows in set (0.00 sec)

mysql> EXPLAIN SELECT * FROM cupid.cupid c WHERE NOT EXISTS(SELECT * FROM cupid.cupid_msgs WHERE receiver_id = c.member_id);                        +----+--------------------+------------+------+---------------+---------+---------+-------------------+---------+-------------+
| id | select_type        | table      | type | possible_keys | key     | key_len | ref               | rows    | Extra       |
+----+--------------------+------------+------+---------------+---------+---------+-------------------+---------+-------------+
|  1 | PRIMARY            | c          | ALL  | NULL          | NULL    | NULL    | NULL              | 1840658 | Using where |
|  2 | DEPENDENT SUBQUERY | cupid_msgs | ref  | PRIMARY       | PRIMARY | 4       | cupid.c.member_id |  226603 | Using index |
+----+--------------------+------------+------+---------------+---------+---------+-------------------+---------+-------------+
2 rows in set (0.00 sec)