The supported linux OS's are listed in the release notes
Are you purchasing a scalix license or sticking with the community edition.
For the purchased version of scalix, Your free OS choices are CentOS 5, Fedora 9 and OpenSuSE 11. Fedora 9 is so old that I would stay away from it, CentOS 5 will be even older and have let packages available. I think the Fedora Project end-of-lifeed ver 9 years ago. OpenSuse 11 is relatively current- which will matter if you want thinks like ClamAv. It may also depend on what version of linux you are more familiar. It doesn't say what Debian systems are compatible with the Comunity Edition but you could look at Ubuntu.
But since Scalix development seems to be on-hold I would caution against a major investment in time or effort in this project.