Spam filters

http://kb.mailchimp.com/article/avoiding-the-spam-filters

http://www.microsoft.com/mscorp/safety/technologies/senderid/default.mspx

http://kb.mailchimp.com/article/authentication-for-mailchimp/

http://blog.mailchimp.com/most-common-spam-filter-triggers/

http://www.inc.com/geoffrey-james/how-to-avoid-a-spam-filter-5-rules.html

http://spamassassin.apache.org/gtube/