Yes. Sorry about that.
Well yes, that, or in some other fashion, configure the mail-receiver (i.e., Postfix) to deliver mail somehow. I’m mostly of the opinion that if you know how to do that then you might rather just do that rather than use the mail-receiver.
Another solution would be to have some <mail thing> process the mail for domain and forward the rest to the mail receiver, perhaps under some other MX.