If the Author Email address field is not set in the Blog Feature, users are presented with an error when they submit a comment. The comment still posts but they get an error. I have verified this on 2.3.1.7 and 2.3.1.8. I am not sure when it started.
The error is: "System.ArgumentException: The parameter 'address' cannot be an empty string." Of course, users see the normal mojoPortal error screen, not the actual application error because CustomErrors is turned on.
-Joe