Solved ! It's not a ClamAV problem after all. It's a Mozilla bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=209501
The 12/31/1969-dated phantom emails are generated when (in my case at
least) voids (two end-of-message characters in a row ?) are created
when the search function is used in Thunderbird (or Icedove) to find
target emails in one folder and then move them to another folder. The
move also deletes those emails from the source folder, but that leaves
voids, and when Mozilla comes back to see if it made the moves of
those selected emails successfully, it finds a number of blanks in
their place, which it proceeds to move again to the destination folder
as though it had missed them in the first select & move operation.
When I have a lot of emails to move all at once (10's to thousands)
then the process repeats a number of times, and Mozilla records a
count of X*(N-1) emails moved. The subsequent passes in this process
go by with bigger and bigger "gulps" of emails (after all, they're
blank !)
There's no getting around this, because the search results popup in
Thunderbird has no copy mechanism that otherwise would allow one to
move the emails from the search results in a manual two-step process:
Copy and then delete.
When those 12/31/1969-dated phantoms are later searched out in the
file structure (one per destination folder is what is always found)
and deleted, the trash folder only displays one phantom
12/31/1969-dated remnant.
The generation of these phantoms is also associated with password
changes recorded in emails received and moved to file folders. A
search on the Internet for "password 12/31/1969" reveals that
12/31/1969 is the zero date for the linux perpetual calendar, and that
this has been exploited by admins to set passwords by some sort of
sleight-of-hand:
http://www.codejourneymen.com/content/adding-admin-user-drupal-site-without-overwriting-admin-user
https://books.google.com/books?id=tj0-8ctawTsC&pg=PA108&lpg=PA108&dq=password+12/31/1969#v=onepage&q=password%2012%2F31%2F1969&f=false
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