FYI there is a perl module (Date::Parse maybe?) for parsing dates out of email that probably does a better job than us
Also, once you have the date, try using the "zmmailbox addMessage" command with the "--date" option instead of trying to add the X-Zimbra-Received header.
Users of imapsize might consider petitioning the developers of that utility to honor the "internaldate" imap flag. FWIW thunderbird 2 now does this.