First of all, everyone's on Exchange 2010 using OWA

I have a dynamic distribution list that contains all users in my domain called 'staff'. I can send to this list, other people can send to this list, but I have one user that cannot send to this list. Sending to this list gives the user an email back with the error:

Delivery has failed to these recipients or groups: Staff

The e-mail address you entered couldn't be found. Please check the recipient's e-mail address and try to resend the message. If the problem continues, please contact your helpdesk.

and then a bunch of diagnostic information that I don't want to paste here because I don't want to have to censor all of the sensitive information contained (lazy)

Can you guys throw me some possible reasons why this would happen? If there are an innumerable number of reasons, where should I start to troubleshoot this?

EDIT

One Exchange server inside the network that acts as a transport server, client access server and mailbox server and one Edge Transport server in the DMZ.