If you are online the SLmail installer can contact Seattle lab and determine whether the version you are installing is the current version. By default, the installer will attempt to do so. If you do not wish to verify that you have the latest version, clear the check box.
SLmail Installation Check for New Version dialog box
If you are connected to the Internet and the installer finds a different version of SLmail on Seattle Lab's website, it will ask you if you would like to stop installation and go to the upgrade page.