Resolution:
A delivery-tracking sender assigns sequence numbers serially for each outbound subject, so the sequence numbers reflect the order of messages from a sender on a specific subject.  When the certified delivery software presents certified messages to a certified listener, it preserves the sequence in which the sender sent them.  If a message arrives out of sequence, the certified delivery software in the listener requests the missing messages and does not present the out of sequence message to the callback function until all the preceding messages are available.