So, this question got my attention, there's an exact duplicate on SF that has already been answered. Migrating it to SF would probably get it closed there as exact duplicate. Should we just close it as off-topic/delete it here and refer the OP to the SF version? Or should we migrate it anyway and let them at SF deal with it?
I'd be tempted to just delete/close it here right away and refer the OP to SF. But should we then just close it as off-topic, no migrating?
EDIT:
And this question is in a grey area really... there's nothing to indicate it's a production server... I mean, I run postfix on my machine... And I've seen loads of questions like those here. I wouldn't per se be off-topic here...
EDIT2:
My greatest con for deleting would not allow for a response to be left for the OP by users (like I did with the link to SF in a comment), so he'll be clueless as to where he might find his answer...
https://superuser.com/questions/243570/how-to-forbid-postfix-to-send-to-external-domains