DaniWeb IT Discussion Community - Small Business Server 2003 SMTP Problem

I have mailboxes set up on my Small Business Server 2003 machine. The e-mail comes in, gets routed through an x.400 connector, and put into the user's box. Here's my problem. I have a user named "Austin," who's username for the server is aochs, and his e-mail is austin@whatever.com. I have the SMTP address associated with his name as austin@whatever.com and austin@whatever.local, which is how all the other ones are set up. But whenever I send an e-mail to Austin's email address from an outside e-mail server (such as Yahoo or whatever), I get this in return:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

aochs@*****.com
SMTP error from remote mail server after RCPT TO:<aochs@*****.com>:
host *****.com.inbound10.mxlogicmx.net [***.**.144.3]:
553 mailbox aochs@*****.com is restricted (Mode:
normal)

Why might I be getting this? His account is set up like the others, but his is the only one that is bouncing e-mails. Any ideas?