A company has a NT4 server running Exchange 5.5 sp4 on a local network with port 25 forwarded to it for receiving mail purposes. The company's IP address matches the MX record. At this point they are not willing to upgrade their Windows server but are concerned about the amount of Spam and virus email they receive.
I would like to set up a Linux box (preferably Debian) to sit on the LAN and take all the incoming port 25 messages, scan them for viruses and spam, and immediately forward them along to the exchange server. I only care about incoming mail, if it is more simple I'd rather just have the exchange server handle the outgoing mail.
I have been reading up on this and I'd like to use Exim with Mailscanner, which appear to integrate really well with Spamassassin and a scanner like f-prot. I do understand how Exim receives mail and scans it, but I have not found any information on how to configure Exim to pass the mail to the exchange server.
Let's say I make the Linux box 192.168.1.20. The MX record points to the router, which points port 25 to 192.168.1.20 (the Linux box). Exim receives and processes the mail... How does Exim at this point know to push the message along to the exchange server (call it 192.168.1.25)? Do I need to create an alias for the domain, do I hard-code the IP address into exim.conf, or what?
I would also to enable the Linux box to poll the exchange server for HAM (legit mail marked as spam) and run sa-learn on it. This isn't necessary but it would be nice.
Thank you for all of your comments.