This software is in parts open source, and is one of the few mail and groupware servers that can serve the needs of fortune 500 companies.
This is not and it never was the realm of Microsoft, this is where Unix and Linux became big. This is about high-availability, stability, scalability and quality. Thousands of users on one server, automatic replication and high-end-clustering.
Furthermore, Scalix is said to have better Outlook support than Exchange itself – if you want to get the full picture, have a free look at it here.