opendkim: Check additional headers to determine sender

Despite the default semantics for `InternalHosts` option in
opendkim.conf, OpenDKIM seems to generally only consider the labeled
sender of a message when deciding whether to sign a message or validate.

This means that previously, when a message was sent to a mailing list
from a domain outside of the mailnode host, the copies of this message
that the list sent out did not get a signature applied to them.

Since we will usually be injecting headers for mailing list information,
we can search these first to determine the message envelope sender.
X-Mailing-List will be the list's full address.  Sender is added just in
case.  Advice is taken from the "Mailing Lists" section of
http://www.opendkim.org/opendkim-README

Signed-off-by: Malfurious <m@lfurio.us>

0 files changed, 0 insertions, 0 deletions