postfix accept remote but don't relay

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http://serverfault.com – I'm trying to configure a basic postfix server to allow receipt of email from anywhere to local addresses but not be an open relay. Current config, seemingly relevant rules: mydestination = $myhostname, localhost.$mydomain, localhost mynetworks = relay_domains = $mydestination If I change mynetworks back to it doesn't receive external mail. Leaving relay_domains blank seems to make no difference and keeps this as an open relay. What am I missing? (HowTos)