Without other details, the addressee's server reports the mailbox you are trying to reach does not exist there (i.e. correct domain, incorrect box.)

If hg.com is your domain, then the only thing I can think of is that some other error was returned by the destination, your server tried to deliver that bounce to the "username" mailbox, could not find it, and then somehow managed to deliver the double bounce to your admin box.

Most of my boxes run Exim, so I'm afraid I can't give any hints on how to check the aliases in PostFix.