遇到SQL JOIN问题

Could someone help me make this better? I am trying to get all rows from both tables where the sender and recipient matches the user_id and trashed has the value of 1; This is what I've done. But my code is returning only one row, whereas there are at least two rows: one on each table. Thanks.

$SQL = "SELECT * 
          FROM inbox 
          JOIN outbox 
            ON inbox.recipient = outbox.sender 
               AND 
               inbox.trashed = outbox.trashed 
         WHERE inbox.recipient = '$user_id' 
               AND 
               inbox.trashed = '1'"