I don't agree that Linux is the best server os. It's the cheapest and more appropriate solution for many applications such as a web server or file server(although Windows is also very good at this). It's not the best for LDAP and for RDMS systems.
This is why you often find most large companies have a mix of Windows and Unix.
Most will use Windows to manage users and mail mainly because its a better fit with their desktops where Windows is the most popular. Companies with a heavy .net use will be better off using Windows server to host their web sites. Most Oracle shops use Unix, despite Oracle themselves pushing their Linux.
This is why you often find most large companies have a mix of Windows and Unix.
Most will use Windows to manage users and mail mainly because its a better fit with their desktops where Windows is the most popular. Companies with a heavy .net use will be better off using Windows server to host their web sites. Most Oracle shops use Unix, despite Oracle themselves pushing their Linux.