Thanks to everyone who responded to my request for help.  I alerted my provider and - sure enough - there was that secondary MX problem that they were not even aware of.  As much as I hate to admit it, I was fairly certain that the fault DID lie with my provider and not my subscriber since the error I was seeing just popped up for no apprent reason, which made me think my provider had been jerking around with things again.

 

Again - thanks for all of the help.  I really appreciate this group and the expertise exhibited daily.