The following points should be kept in mind to improve your email deliverability:

  1. Remove inactive contacts who don't open your emails. It's very important to routinely clean up the inactive subscribers. You should only be sent to contacts who have opened in the past 12-24 months. Make sure you're constantly adding new subscribers to your list. 
  2. Focus on good, engaging content that your contacts will find interesting. If you repeatedly send purely promotional, hard-sell offers, you will see lower open rates and click rates.
  3. Send more targeted, triggered, personalized emails. It's a lot more likely for big batches of mail to be identified as spam. When possible send fewer, more personalized messages.
  4. Sending frequency should be a once a week or fortnight. If you send too often they may mark your email as spam. 
  5. Pay attention to the spam filter testing before sending it. 
  6. Don’t send a single graphic/image as it's a sure-fire way to have delivery issues. On the flip side, it's not necessarily better to send plain text emails. The general rule is to have a good, balanced mix of images and text.
  7. Use an email address for the company or organization for which you are sending an email from.
  8. Pay attention to your links in your email. Spam filters check the URLS that you are linking to. 
  9. Don’t use link shortening services like bit.ly. For the outward-facing text, it's better to use “click here” or a button.
  10. Send using a consistent also, from name should identify brand clearly and email address. Keeping it consistent can help build your reputation.
  11. The subject of the mail should be different every time. 
  12. Make it very easy to unsubscribe through an unsubscribe link.

Keep in mind that your primary focus should be to maintain a healthy list of contacts, send content they truly want, engage your contacts, and encourage them to interact with your campaign. If you keep your focus on these items great delivery will follow.