You can make a UNION SELECT here. OnlyThe only problem is to match the columns from messages, but you can guess those by adding columns until it fits:
SELECT * FROM messages WHERE unread = 1 LIMIT
1 UNION SELECT mail,password,1,1,1 FROM users
Just keep adding ,1 until you get the correct column count. Also, you need to match the column type. Try null instead of 1.
If you can see MySqlMySQL errors that would help big time here. Otherwise you got a lot of trying.
Also see owasp.org Testing for SQL InjectionTesting for SQL Injection at owasp.org for some details.