The TMessageRead structure

 // MessageRead
 TMessageRead   struct
  Result                sbyte  // The function call result. NO_ERR means it succeeded.
  QueueID           byte   // The queue identifier.
  Remove                byte   // If true, remove the read message from the queue.
  Message     byte[] // The contents of the mailbox/queue.
 TMessageRead   ends

See the Mailbox constants group for QueueID values