Outlook- pop3 client using external email in my domain

[encinitas]

I have a user who is only using mail from Register.com and uses Outlook 2003 as a pop3 client. But he can't send or receive anything from any of my internal users. (while connected to the LAN)
This seems to be an authentication problem that my Exchange box doesn't like to be a relay for this user- or something along those lines.
Is there a setting to allow this, or some other solution to let him get his mail while sitting on my network, using the Outlook client?

thanks in advance.

 

[encinitas]

Looks like the user is able to send mail to one internal user. The rest get bounced back. Anyone have an idea on this?