Fatal Error to undefined function wp_mail

My website visitors are getting the error below after entering an email in the coming soon notify me box:

Fatal error: Call to undefined function wp_mail() in /myhost_path/wp-content/plugins/easy-coming-soon/template/coming_soon_display.php on line 183

I believe others have also reported this issue with the plugin. There definitely is something buggy that you need to take a closer look at.

I have dialed and reactivated the plugin with no luck

Thanks

http://ww.wp.xz.cn/plugins/easy-coming-soon/