That reminds me that in 1999, looking through the referrer logs, I realized that if the link came in from an Outlook email, Outlook+IE would report the subject

That reminds me that in 1999, looking through the referrer logs, I realized that if the link came in from an Outlook email, Outlook+IE would report the subject of the referring email as the referrer (iirc with user name, something like “mailbox://user@site/subject-of-the-email”). So we started looking for those more seriously in my company, and got quite a bit of interesting Intel from potential investors, competitors we knew about, and some we weren’t even aware of. It was just the subject and user, but was often surprisingly informative.