Email Settings should be mandatory during installation or email related features should be disabled until "Email Settings" is done. Otherwise, it will definitely confuse the admin of the system as nothing is really work because user cannot be created successfully without email activation.
I think it would be easier to firstly disabled it to get it passes the installation process. Since the installation process is already nice and easy, forcing the user to configure email settings would be a bumper. Then, check if the email settings have been configured before allowing the admin to add new user, so that a proper warning message can be shown to the admin that he needs to set the email configuration first.