1. In some situations, Windows interprets a semicolon or comma in a folder name as a delimiter–in other words, it will assume that the folder name is part of a list of folder names. This can lead to dialogs (like the Open and File As dialogs) reporting that they can’t find the folder you are looking for. So probably the semicolon is a good character to omit in folder names, even though Windows permits them.

2. In addition, the characters [ and ], while they are permitted in folder and file names, sometimes get Windows confused. If a folder has one of these characters in it, the File Save dialog will sometimes complain that you shouldn’t use the characters <> ? [ ] : | or * in a file name (actually, this should say path, not name). All of these except [ and ] are generally blocked when you try to use them, though the error message suggests that there may be ways of sneaking them in anyways.

So the remaining special characters that are neither prohibited nor problematic for use in both Outlook and Windows folder names are: ! # $ % & ( ) @ ^ _ { } + =. All sort to the top of the list, in the order shown.