When naming a folder in your mail file, Notes will propt you if you try to duplicate a main folder name on a new mail folder name, but it allows it if the duplicate folder name will reside in a sub-folder. Since this will eventually cause problems after several duplicate folder names are created, Notes should not allow any duplicate folder names. Problems are also evident when the user uses punctuations and/or symbols (e.g., ! @ # $ %  ^ & * (  ) + = , . / ? | \ { } [ ] ` ~) in their folder names... and yes, I have seen them all. Using symbols can disrupt the indexing of the folders. Therefore, it would make sense to not allow the user to use these when creating new folders.