suddenly, i can’t send e-mail

I’m running Exchange 5.5 with outlook 2000. my LAN exchange clients can send/receive fine. My remote internet clients, those who use other ISP’s to access the Internet, can receive pop3 e-mail from my exchange server, but none of them can send e-mail. This just started happening a few days ago. Here’s an ex:
laptop user running win2k/outlook 2000/earthlink isp. pop3/smtp servers are our exchange server. i can receive, but can’t send. when i change the smtp server to any other (eg: earthlink, yahoo, etc…) it works. again, my LAN exchange clients can send, but my remote can’t. any ideas?
p.s. the error i get is “the server was found, but there was no response…” and “unable to connect to the outgoing mail server (SMTP). Please verify the server name….”