remote e-mail

Hello PLUG,
I have remote users sending e-mail thru
my linux e-mail server. We are using the
pop-before-smtp (DRAC) to authenticate the
users ID before allowing relay.

However, one user connects through COX isp
and he cannot send through. He can pop his incoming
mail but gets an error: "Unable to connect to the server"
when sending out.

I do not have this problem from other isp's only the COX

Does anyone else had problems with COX.

Thanks,
Dan