How to join 2 tables in mysql but with different values in one of the tables
You can use LEFT JOIN instead of JOIN.

With LEFT JOIN the query will return all records from the left table i.e. the one in the FROM, even if there is no matching record in the joint table.

SELECT * FROM inbox_messages_table i 
LEFT JOIN users_table u ON i.from_user_id = u.user_id 
WHERE u.banned = 0 AND to_user_id = $user_id AND deleted = 0 ORDER BY message_timestamp DESC