What causes a bounce message that says,"Desination server rejected contents"?

That "Destination server rejected message contents" error is one that our servers generate when they've successfully processed a message and attempted delivery, but received a permanent rejection from the recipient's mail server after the DATA phase of the conversation.

In other words, something within the contents of that particular that message set off a rule within the mail server's secondary spam / virus filtering.  The filtering servers do not have any visibility into what specifically triggered the destination mail server to think that the message contained spam or a virus.  One common cause is that the message contained a Microsoft Excel document that was detected to have macros enabled; many server-based antivirus software brands will automatically block such files.  To determine the cause, it may be necessary to review the logs on the receiving mail server.