1. Domain should not be black listed.2. Don't use whole image as a templete.3. Avoid following words in your mail : prize, free, bonus, buy, sales, purchase, order, $, money, lottery etc.4. Always an unsubscribe link should be present in the footer of all our emails.5. Keep your list clean with valid emails (If u send mail and it will bouns back then plz remove thoes mail id ).6. Don't use lots of colored fonts.

http://www.activecampaign.com/help/improving-delivery/

http://mailchimp.com/resources/guides/how-to-avoid-spam-filters/html/

https://support.mailpoet.com/knowledgebase/6-golden-rules-to-avoid-being-labeled-a-spammer/

https://sendgrid.com/blog/10-tips-to-keep-email-out-of-the-spam-folder/