mbox ownership

As root, I process several unix mbox files which belong to non-root
users.

How can I force MailBox to preserve the correct ownership of the boxes ?

I read an answer from Mark stating that a call to chown could restore
the correct ownership.

Yes. But AFAIK, this call is only possible AFTER the $folder->close(),
which unlocks the box, giving to a waiting sendmail process the
opportunity to throw away the "BOGUS mbox". 

Is there a "flush" function somewhere which could allow me to call chmod
BEFORE the close() ?