Operating systems provide the basis for network-available servers, such as email servers. But aside from the normal email security concerns in the server itself, there are a plethora of security concerns within the operating system. Microsoft Windows, once proven to be fairly insecure without numerous patches and updates, is now considered robust and secure. Linux, or other flavors of Unix, also makes a good choice for email servers.
But aside from understanding that these servers need a strong foundation in a secure operating system, you also have to be able to afford and support these servers. For example, Linux is often a free choice, costing very little for initial implementation. Microsoft can cost your business a lot more money for both its operating system and its email server software. Which of these can you afford in your budget? Which of these choices can best be supported by the current skill sets at your organization?