Comment 5 for bug 456238

The explanation for this issue is in Postfix 2.6.5's release notes (http://www.sfr-fresh.com/unix/misc/postfix-2.6.5.tar.gz:a/postfix-2.6.5/RELEASE_NOTES):

240 [Incompat 20090330] Postfix now adds (Resent-) From:, Date:,
  241 Message-ID: or To: headers only when clients match
  242 $local_header_rewrite_clients. Specify "always_add_missing_headers
  243 = yes" for backwards compatibility. Adding such headers can break
  244 DKIM signatures that cover headers that are not present.
  245 compatibility with existing logfile processing software, Postfix
  246 will log ``message-id=<>'' for messages without Message-Id header.

It is therefore intentional and the bug can be closed.