TL/DR; Your users may get a ‘There Has Been a Critical Error on This Website’ message when they submit a comment
It probably means your webhost has disabled the PHPMailer function on a shared platform for security reasons. The fix is simple: download an SMTP plugin and use your website’s SMTP server and one of its email accounts and you’re good to go.
NB, although your users get that scary message, their comment may still be posted on your website. The point of origin of that message is your website sending the notification to you not something your user is trying to do. But best fix it, eh?
Big shout out (because that’s what us kids do) to Masher for spotting this error and to me for fixing it.