It probably means your host started verifying that the sending email address actually belonged to your site which means if you use an off site email address such as your local email address, Gmail, Hotmail etc, the mail doesn’t get through. I posted about this awhile back and there’s a small plugin that was worked up by a WordPress developer to work around the problem. Works for me just fine and you can download the zipped up plugin from my site (I got it from WordPress Trac). This will enable you to continue to use a non-site related email address for notifications.
https://just-thinkin.net/2007/11/wordpress-231-no-email-notification-fix/
Also, you can solve this problem by going into your hosts cPanel (or whatever) and creating a new email address like this: “wordpress@(yoursitename.com/net/???)” and use that one for notifications instead that way no plugin is required.
Hope this helps.