Why are my emails classified as spam?
The very first thing to check is if you have a dedicated or shared IP address (i.e. IP address is shared between several or more domain names): http://www.yougetsignal.com/tools/web-sites-on-web-server/.
Another solution that could help is sending emails via SMTP. There are many plugin options available for WordPress: https://wordpress.org/plugins/search.php?q=smtp.