source :
- https://support.google.com/a/answer/33786?hl=en&ref_topic=2759192
- http://www.x-pose.org/2013/10/22/how-to-designate-an-ip-address-as-permitted-sender/
- http://ericbrandel.com/2011/12/14/email-deliverability-spf-records-part-3/
- http://www.cyberciti.biz/faq/bind-named-djbdns-google-apps-sender-policy-framework/