Jerry - yes, you can specify the full address yourself because you are an administrator and have that ability. However, a normal user would not have that ability and cannot modify the email to address field - which defaults to only their username.

If you want to give your users the ability to setup and create subscriptions - then you have to specify the default domain. Or (I haven't tried this myself), you can disable the property right above that 'SendEmailToUserAlias'. Not sure if that will work - but, something you can test.