550 unknown user (known user though)

I work at a hosting company. One of our many clients as their emails
set up through us. We are finding though that one of the mailboxes
can not be emailed internally from their office yet I can email them
just fine and they do recieve it. Anyone delt with this before or
have an idea what could be causing it? Thanks for any help!

Here is the bounce back one of their employees is recieveing when
sending to this one mailbox. (checked, double checked, and triple
checked spelling)

------------------------------------------------------------ -------------------------------------------------

Your message did not reach some or all of the intended recipients.

Subject: FW:
Sent: 11/1/2007 2:55 PM

The following recipient(s) could not be reached:

'x@xxxxxxxxx.com' on 11/1/2007 2:55 PM
550 unknown user