with commit 7d4a3ea
there was an DefaultMailPrecedence config option introduced,
which is set in RT::Action::SendEmail::SetRTSpecialHeaders.
Beside this isn’t a special RT header, dashboard and digest mail
(sbin/rt-email-dashboard and rt-email-digest) will not have an
default precedence header because they call RT::Interface::SendEmail.
So it is maybe better to move the setting of the default precedence header
from RT::Action::SendEmail::SetRTSpecialHeaders to