By default someone can comment on your site using your site’s member’s email. Comment form has email text field, one can just type in an email of your site’s registered user and post comment on your WordPress site as him.
But you can prevent unauthenticated users from commenting by using a registered user’s credentials. All you need is install and activate Impostercide plugin and That’s it.
Now next time someone trying to post a comment using registered user’s email, he’ll be getting this warning message.