Thank you for writing in, that is just a warning and won't affect the functionality of the plugin. You can disable the WP_DEBUG to get rid of those warnings by following the guide provided here.
However, we do have a real bug on Email Form for now (check this thread). I advise using an alternative instead you can try the ConvertPlus extension or if you're using MailChimp service try this plugin MailChimp for WordPress.
You can check our Changelog page when we release an update to see if a fix for Email Form extension is included.