There are a couple of rules that will help you avoid getting redirected to a person's junk/SPAM folder.
- Avoid words like: Free, Win, Cash, Bonus, Membership, Free Offer, Call Now, Rates, etc.
- Avoid use of CAPS.
- Avoid excessive punctuation eg. !!!, !?!, ???
- Avoid all image newsletters. No text makes it hard to detect content purpose. Balance text with images with a ratio of around 75% text / 25% images.