Thanks for the response.

I have tried resetting the Messages view on the Inbox. There is no
effect - the messages remain hidden.

If I switch to other views - say "Last 7 Days" or "Unread Messages",
then the messages appear, in the same way that they do if I click away
from the Inbox to another folder (such as Unread Mail Search Folder)
and then back to Inbox.

I have completely disabled NIS (email scanning and spam filtering) to
no avail - the problem persists.

Thanks again,
Gomer