Title: Require Visitor to Enter Email When Commenting
A "Visitor" is a user who can access your server, but has not logged in. If the Visitor account is given comment permissions, the Visitor will be have the option to include their Name and Email address. (note, however, all comments made by a Visitor will show as Authored by "Visitor" - the Name and E-mail address information will appear in the comment footer).
When this setting, "Required Visitor to Enter Email When Commenting" is set to "yes," the user will have to provide a value in the Email field.
There is, of course, no assurance that people using the Visitor account will enter their real email address, but this does require that a value is provided.