The email address in the FBA Site Configuration page is just the reply to address that will be on all emails sent from the FBA Pack.
One thing to check - are the emails you're trying to send to external emails (gmail.com...), while the emails you tested are for internal emails?
If that's the case, your smtp server probably isn't configured for relaying. That will have to be setup. Test setting up a user with an internal email address to see if that works.
If you still have issues, check the SharePoint log file - it should tell you the error it encountered when trying to send the email.