A few key steps on how to avoid the spam filter...

1, Understand and respond to complaints. (It’s all about setting expectations).
2, Ensure email database is up to date. (Avoid sending emails to dormant addresses. Keep the list clean).
3, Infrastructure and Authentication. (Home-Grown systems are more susceptible to this. ESP’s manages this daily).
4, Avoid switching your IP address (ISP's are aware of this practice and may mistake you for a spammer)