One internal user cannot send email to another

I have an small internal domain of 14 users and everyone in the domain can send email  internally and externally except one user and gets the  error:

IMCEAEX-_O=MYDOMAIN_ou=first+20administrative+20group_cn=Recipients_cn=al@mydomain.net
#550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##

Everyone is using Outlook 2010 and this is the only issue to date...The really sad part is I'm the IT guy and one user cannot send to me. They have to resort to sending to my Gmail account. My thoughts are why can everyone else send to me but not this one user?

This was a fresh install on a brand new server.

Thank you in advance