type:
constant
name:
USE_PRECEDENCE_HEADER
purpose:
Use the Precedence: header in outgoing mails
allowed:
true false 0 1
default:
true
category:
rendering

There is a debate whether or not this should be used, and it seems different per major ISP.

Amazon SES will not work when used, but Gmail likes it to be there. So, experiment and check your own results and then post them back here, or in the forums, so that everyone learns.