If you're looking for a Linux replacement for Exchange, there's 1 that I know of. Well, 2, if you consider the secured appliance. Allow me to explain:
Bynari ( www.bynari.net ) makes a linux replacement for MS Exchange called "Insight Server" that was designed mainly to run on RedHat and Debian GNU/Linux.
Another company, Immunix ( www.immunix.com ) takes Insight Server and hardens it into a "Secure Appliance." Setup and configuration is a snap (via a web interface) - from booting from the CD-ROM to a working server is literally 10 minutes.
The reason I know this much about both products is because I'm a certified partner/reseller for each company. Mind you, I won't make any money by suggesting that you look at them (or even if you decide to go with one of them.) The reason I decided to sign up with each company in a partnership is because I've used both products, and find both of them to be outstanding performers.
There is one other MS Exchange equivelant that runs on Linux, but it's not a standalone product, meaning it's part of another solution. (Contact me privately if you would like further information on it.)
I hope some of this has answered your question. Keep in mind there are other Linux applications that claim to be on the same par with Exchange, but do your homework and read the reviews. I've tried about 14 other applications and finally settled on these two for Exchange replacements based on performance and security.