I can’t agree as I have only experience with Windows 2000/2003/2008 and exchange. But I think you need to look at the entire envionment before judging. From the edge all the way to the server you need to provide protection and maintenance following industry best practices. My fully patched 2000 exchange server has run flawlessly and with no security or operational issues since it was built 4 years ago.
You mention you are using Red Hat Linux for your mail server, but what MTA, MDA and user access method (POP, IMAP) are you using? That makes a difference more so that just the OS. Have you made sure that your mail server is not an open relay? Is the MTA running as a non-priveleged user? Does your mail server touch the internet or is it only internal?