How do I configure hMailServer for relaying mail sent to local SMTP onward to GMail?

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http://serverfault.com – I have just installed hMailServer to enable me to test my PHP code wants to send mail using smtp on localhost port 25, but some parts of the setup procedure and instructions1 are not clear. Guides say, "Add a domain localhost.localdomain". This seems to be horribly over simplified. What domain do I add? I at least need some DNS here, even if it's just host file based. I want to relay outgoing mail to my Google Apps account at smtp.gmail.com. I have set up my account settings in the hMailServer SMTP Relayer settings, but may I assume that all mail without an account on my local test domain w (HowTos)