[cobalt-users] Mail relay on Raq550

["Ben Fitzgerald-O'Connor" <Ben.Fitzgerald@xxxxxxxxx>]

One of our clients is in the process of installing their own mail server (Exchange), and will be having email delivered via SMTP directly to them, instead of using the POP/IMAP/Webmail that they currently access on the Raq550 their domain is hosted on.
 
We would like for the mail to still be routed via the RAQ, so that it benefits from a relay, in case of their main server being unavailable, and further benefit from the virus scanning package on our Raq server {full marks and a hearty recommendation for SolarSpeed, who have very efficiently set up their AV-Suite package of Kaspersky & Amavis on the box}. 
 
What configuration changes to we need to make in order to allow the mail to be relayed through the server for these domains? We plan to keep the server as the primary MX record, and put an entry in the /etc/host file, so that it will then resolve the lowest MX record as an address other than itself (the final destination). If anyone could advise as to the changes we will need to make to the sendmail configuration, it would be most appreciated.
 
The server is currently set to only relay if Pop authenticated first, and we only want to relay for specific domains, not to inadvertantly create an evil open relay. We are currently getting a bounce message back from the our server saying ' no such user here', which makes me think the system is looking for a local recipient, not attempting onward relay.
 
Many thanks in advance for any help / advice you can offer. I've had quite a long hunt through the archives, and the Sun knowledgebase, but unless I've missed the obvious, I can't find the solution (though I'm certainly only just learning about the intracacies of Linux).

Regards,

Ben