Mac OS X is an evolved version of the Macintosh operating system. It is actually a complete rewrite and is based on the NeXT operating system that Steve Jobs (founder of Apple) helped create in the 80s. Instead of being based on previous versions of the MacOS, it uses a different kernel and is now based on a BSD unix platform. This gives it more stability and abilities including something which it always lacked. A command line shell.
At Suso we support and encourage use of Mac OS X. We would discourage the use of Mac Mail, because it can cause problems with our IMAP server.