The origins of X-Priority
As the Internet moves further and further into the future with high bandwidth, low latency broadband connections, it's easy to forget where things came from, and why the hell they're done the way they are. A recent discussion on fedora general regarding Evolution's lack of "message priority" functionality lead to this post by Fritz Whittington:
It's the small things we often overlook, these days most email rants are over top posting, HTML posting, thank god the days of "wait untill long-distance calls are cheaper" are over for mail delivery...Well, if you open the horse's mouth, and look at the teeth, you'll note that there are two commonly used headers: X-MS-Priority:
and X-Priority: . The former was of course invented by MS. The latter was invented much earlier, when mail actually went from one machine to another to get cross-country, sometimes on dial-up uucp links. And it was intended to priortize the transmission of mail, therefore more for the use of the MTA's than the users. In particular, X-Priority: 5 was used to indicate mail that had long lead times (like announcing a conference 6 months in advance). This mail could obviously be saved until the wee hours of the morning, when long-distance calls were cheaper. X-Priority: 1 was intended for extremely urgent mail, usually having to do with network conditions, and generally sent between sysadmins. The net was so different then, no one would dream of setting a bogus priority that was inappropriate for the real urgency of a message.
And they aren't really NON-standard. All headers beginning with "X-" are allowed specifically by the standard, with the usage and semantics to be agreed upon by the community involved. That's why when MS wanted a *user-to-user* indicator of priority, they avoided the earlier X-Priority: header and set up a distinct one for their own "community" of Outlook users.
interesting Mark - thanks for your insight