You phone them and ask. Short of hacking into their server to find the underlying script/code that delivers the comment, you're out of luck. Bear in mind that MANY (if not most) online feedback forms don't even have email behind them, they submit forms to CRM applications or trouble ticketing and support applications where an engineer/helpdesk person is allocated to deal with the query. I am not sure why you would ever need to know what the underlying email address could be, but chances are you will gain nothing by knowing.