I want to achieve the following with postfix:
Almost all outgoing emails shall be sent via a relayhost, configured in main.cf with
relayhost = my.mailserver
There's one specific email address (email@example.com) for which I want the email to be delivered locally. I set up a local account on the machine to match the email address and added "mydomain.com" to the mydestination property in main.cf. Then of course postfix tries to deliver every email to @mydomain.com locally and bounces them because the respective user doesn't exist.
Question http://serverfault.com/questions/149453/hybrid-gmail-mx-postfix-for-local-accounts seems to be somewhat related (though more complicated).