550 Relay Prohibited..

My Exchange 5.5 (SP4) is sitting on the same m/c as MS Proxy 2.0. It have 2 NICs, one Internal IP and one external IP respectively. Exch svr enabled “Reroute Incoming SMTP Mail” and Routing Restrictions for “Hosts and clients that successfully authenticated”. Internal client could send and receive mails. However NOT for external client or host even though they has entered their account id and pw in their Outlook->Tools->Accounts->Server Tab->Incoming/Outgoing Mail Server. How could I enabled remote client or host relay mail and yet restrict unsolicited host or client from using exch svr for relaying mail.