I apologize to any list members for my profligate posting this
evening.  I've discovered the cause of the problem thanks to Douglas
Palmer's helpful email response. I'm writing what I've noticed here
just in case someone in the future searches the list archives for
this particular problem.

To wit: readers of my list reading email using pine noticed that email
coming from the list was not being marked as New or Unread when they
received it.

Douglas pointed out that Pine uses the Status: header, which certain
mail user agents affix to emails for internal record keeping. (In
answer to my own flurry of questions in my last email, obviously,
Status comes from mail user agents and not from sendmail or popd,
etc.)

The solution came after establishing this for myself with a little
play. Naturally, the detail that I omitted was that this particular
list requires moderation of postings, the mechanism of which is to
"redirect" a message back to the list's address from an authorized
email address (i.e. an editor, owner, or moderator.)

If you can't already see the problem, it happened that the mail agent
I used from the account I used for moderating the list (the Horde
project's IMP webmail client as it happens) sends "Status: RO"
information back to the pop server it's speaking with. Thus, when I
redirect the now-approved message it contained a Status: header
indicating that the message had been read.

A solution is not immediately obvious to me, but I'll play with things
to see if one presents itself. I hope this mailing helps someone else
in the future, and thanks to all the list members for their patience
with my verbosity. =)

Pratik Dave