I've got my linux mail clients talking to the MS Exchange server and sending internal mail and replies. But when I try to send an external e-mail, it complains about error 550 - cannot relay mail.
Is this because my linux box is in the wrong domain? or am I missing something completely?

Both boxes have to be in the same domain, or on your Exchange Server go into the Administrator program. Under configuration, go to Connections. Double click on Internet Mail Service. On the Internet Mail tab, click on E-Mail Domain and add the domain there. Then click on the Routing tab. Choose the second option, Reroute incoming SMTP mail. Click add, and add the other domains. Choose route to inbound.
That is how I set up my server to recognize our different domains. YMMV.
Felicity

I am having the exact same problem.

I know this may sound like a silly question (I am still a newb at this) but just because I have my Linux box set up so that I can receive email from Exchange, does that mean it is in the same domain or a different one? How can I tell? I can receive internal and external mail just fine but any outside mail I try to send comes back undeliverable as if the Exchange server were looking for the email address locally (part of the domain). Thanks in advance!

The Exchange server is probably set not to accept email forwards from other domains. This is to prevent spammers from using your Exchange server to forward their spam. What that means is that both the client and the Exchange server need to be on the same domain in order for the client to send Internet emails. So if your server is exchange.yourdomain.com, then your client should be hostname.yourdomain.com. If it is any other domain, the Exchange server will refuse to send the emails. If it is like mine, it will forward refused emails to the administrator. This alerts the administrator to people trying to use the server for spam. To get your client to work, find out the Internet domain name for your Exchange server and change your client domain to match.

Thanks. That does make a lot of sense. The only question I guess I have now is how do I make my Linux box part of the NT domain? I have all the IP/Domain info I need but I am not all that familiar with Linux yet. Thanks again!

I finally got it to work by configuring message routing in Exchange. I had to configure Exchange to allow my Linux client the ability to relay email (specified by the Linux box's IP/subnet mask).
I was doing that and still couldn't get it to work until I found M$ knowledge base article Q258140 that said to change the last octet of the client's mask to match the last octet of the client's IP address.
For whatever reason that did the trick and it seems to be working fine! One step closer to shaking my M$ dependency!

Thanks for the help!!!