Update header to make e-mails more spam resistant

[walder]

Investigate these fields:
List-Unsubscribe:
DKIM-Signature:
Messages should indicate that they are bulk mail, using the 'Precedence: bulk' header field.

[frog32]

link to the things mentioned above:
http://support.google.com/mail/bin/answer.py?hl=en&answer=81126&ctx=cb&src=cb&cbid=bdz93vczcr6z&cbrank=2#authentication