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Another entry tagged "Er, I can't believe I didn't know that" with a side note in "bleedin' obvious".
To arbitrarily restrict which domains a postfix instance will send to (when receiving email via SMTP) you can simply add the following lines to your main.cf:
relay_domains = example.com,lunarlamp.co.uk,google.com smtpd_recipient_restrictions = reject_unauth_destination
I promise to post some interesting stuff soon, rather than these banal little snippets!