> And oh, there's no standard specifying the possible values for this
> header. I've mostly seen "urgent" and "high" used, and of course
> "bulk", "junk" and "list" for list emails.
It is well documented in the SENDMAIL INSTALLATION AND OPERATION GUIDE.
5.7. P -- Precedence Definitions
[...]
Pfirst-class=0
Pspecial-delivery=100
Plist=-30
Pbulk=-60
Pjunk=-100
[...]
I don't think any other MTA than sendmail actually uses the Precedence:
header. As such, this header is effectively useless on non-sendmail
systems, unless people (ab)use it for message prioritisation at MUA level.
[ sendmail uses the Precedence: header to assign queue priorities, so
that, for instance, message with Precedence: first-class are sent out
before messages with Precedence: bulk, typically mailing lists. Get
the full scoop in the sendmail operations guide, Section 4.3 Queue
Priorities. ]
