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.
• 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 like 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.