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/

In case you have a shared IP address, then a potential reason could be that the email's sent by other websites that use that same IP addresses have been flagged as SPAM. If this is the case, then the IP address became untrusted, and you should contact your hosting provider and request a dedicated IP address for your hosting account / domain.

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.