Thanks for your comments, Eric.  I may have gained an additional insight
into this problem.  The original problematic message was sent jointly to
several addresses, including my list, and the subscriber just sent me some
bounce mail she's received which flagged the erroneous way in which she
addressed the message, specifically:
 
To: [log in to unmask], [log in to unmask],
        [log in to unmask] (rogerburns,[log in to unmask])
 
Note the mistaken use of parentheses with a parsing comma in the middle
of the phrase.  I'm guessing that this played a role in the snafu.  I
originally found it odd that there would be any problem since the
subscriber had posted many times before without incident, and her node
(aol.com) is the source of many list messages which hitherto had shown no
similar problems.  I'm sharing this so that if others experience similar
problems, perhaps this experience will shed clues.
 
-- Roger Burns    [log in to unmask]