It's the opposite. E-mail addresses point to the mailbox. With DreamHost, each mailbox has a primary address. You can add aliases to forward to the primary address. So one can either change the primary address or setup an alias. I would setup a second mailbox whose primary address is that of the your domain, and once mail to that address can be sent and recieved, change the primary address of the first mailbox, and setup the subdomain address to forward to the primary address. This is not done automatically.
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