So here’s the deal: Outlook 2003 doesn’t include the Message-ID header if you are sending email through a mail server other than Microsoft Exchange. There are other rants about this:
And they both point to this priceless news bulletin, which contains this gem:
According to Mark, Microsoft apparently had a few complaints from people using Outlook that their machine name was “leaked” in the Message-ID header. Instead of ignoring the complaint or making the host name used in the Message-ID header configurable, Microsoft chose to remove the Message-ID header.
Technically, Outlook 2003 is still