In the previous slides, we discussed features which are available to all messages, whether they were read from STDIN, created as object, or stored in a folder.

However, message objects which are stored in a folder require to maintain more facts than messages which are not related to any folder. For instance, folder messages know their location. Folder messages can be flagged for deletion, which is an useless idea for single messages.

Then, folders can be roughly grouped in three categories: those who store multiple messages in one file, folders which store all messages in separate files grouped in one directory, and folder types which do not provide direct access to the message's content.