Linux

The first question is how secure does the Exchange server need to be? If it is on your internal network then you can relay for the domain that the Linux servers are in. If it is on the DMZ you have to be very careful about doing this. The Linux servers' domain is the domain they are in for DNS (both forward and reverse records need to exist). If the Exchange server can not relay for this domain then could you make this domain be the same as the one that the Exchange server is in? If the Linux servers aren't needed for anything else this should be OK. They can actually exist in two different domains if they need to. The reverse record must point to the entry in the domain that is to be used for mail. These methods allow you to use the tools you already have in house and just reconfigure some things. Another idea is that you can continue having the email go through the Linux mail server, but simply to forward to the Exchange server (this is a waste of a server, but perhaps you can have the Linux box do something else as well). The exchange server simply needs to be configured to accept relays from the Linux mail server only (not domain dependent, can use IP address) or recognize the Linux mail server as a mail server in its own mail domain. I hope this gets you going in the right direction. Good luck.