Sometimes, specially on the new sites or localhosts, you will see the warning shown on the picture above.
That error is triggered because something on your Joomla site tried to trigger the mail functions but they are not properly configured on your server.
You can typically see this error on XAMPP-like and Windows localhost servers where mail functions are disabled by default
If you, for example, install sheer Joomla on the XAMPP-like localhost server and try to register new user, you will see this error, because mail server is not configured by default on XAMPP-like localhosts.
If, per chance, you see this error on the live site, you immediately need to contact your hosting provider to check on the email server configuration.