Welcome email editor stopping emailing

Hi there – I recently took over admin for a WordPress website and don’t know much about WordPress. The ‘Welcome Email Editor’ suddenly stopped producing automated emails for new logins / password resets etc a few days ago, returning the ’email could not be sent, your site may not be correctly configured to send emails’ error. Godaddy support have established that emails can be sent from the site (they put up a test page) – so it seems to be an issue with the plugin. Any hints on how to debug the issue would be welcome (would that be the post_data tag ? and where do you put it ??) ??

The page I need help with: [log in to see the link]