Exchange 2007 and external SMTP connections   

I just put in a new Exchange 2007 server for a customer, but we're having 
some issues with our external users.  These users have been setup in the 
past to POP their email and send their email through the same mail.xxxx.com 
server.  On Exchange 2003, all I had to do is 'authenticate on outgoing' any 
email sent out and all was well.  However, on Exchange 2007, I cannot for 
the life of me get these users to send email from the outside, routing 
through the Exchange server and back out.  Due to restrictions of their ISP, 
I cannot route them out through that service.

When they try to send out, they receive the message:

The message could not be sent because one of the recipients was rejected by 
the server.  The rejected e-mail address was 
'randomexternalperson@randomdomain.com'.  Subject 'Anything goes', Account: 
'mail.domain.com', Server: 'mail.domain.com', Protocol: SMTP, Server 
Response: '550 5.7.1 Unable to relay', Port: 25, Secure(SSL): No, Server 
Error: 550, Error Number: 0x800CCC79

Could someone offer me some assistance on this?

Stephen

date: Thu, 6 Dec 2007 10:31:23 -0700   author:   Stephen Harrington