Before hackers commence an attack, it is almost certain that they will spend a large amount of time researching their target. When hackers want to attack a server, one of the first things they will do is use network scanning tools to obtain information about what OS is being used and what services are running on it. Once the hackers know your OS version, it becomes easier to attack the server because they can narrow down the possible exploits. The more information they can obtain about your server, the easier their job is.
Simple mail transport protocol (SMTP) is one of the most used and exploited services. Using network scanning tools, hackers can easily see what mail service version you are running. In order to hide this information from scanning tools, certain software has to be used such as MetaEdit, which changes the banner shown to a hacker when they scan the server.
Version info usually is shown by default in the banner. This information is visible to anyone who has a network scanner and scans your server. Changing the default banner is an easy way to make it harder for hackers to attack your resources.