Email Best Practices

Below are general settings to avoid Spam Filters.

• Maintain a good IP reputation. Your sending IP reputation is based on many factors, including (but not limited to) bounce rates, blacklisting, spam complaints, lack of sending IP history.
• Avoid Interactive Content (Forms, etc)
• Don’t Use Spammy Formatting (i.e. writing in all caps; using exclamation
  points in the subject line; mixing letters, symbols, and numbers to spell words; using several font colors, sizes, or styles; using large font sizes)
• Keep the Image-To-Text Ratio Low (too many images or too large of an image compared to text in an HTML email can make your message look spammy to a filter)
• Be Careful with Links.
• Avoid Click Tracking unless you know you’re whitelisted.
• Include Physical Address and a Way to Opt Out.
• Don't use spam trigger words and phishing phrases (WIN, Free!)
• List hygiene is key to getting past spam filters. If you send emails to unengaged or invalid contacts who ignore your emails, filters assume you're sending spam.
• Never use deceptive headers, From names, reply-to addresses, or subject lines.

You can check if your mail server IP is listed on some Spam Blacklists: MX Toolbox, DNS Stuff, Sender Score.