Message Folders are used to organize your email messages. By default, all message folders are stored in the directory Mail, which is created in your home directory. When you first start Kmail the inbox, outbox, sent-mail, and trash folders are created. These folders each have special functions:
inbox: where Kmail puts your new messages when you ask it to check your mail.
outbox: where messages are put while they are waiting to be delivered.
sent-mail: contains copies of all messages that you have sent.
trash: contains all messages that you have deleted.
You may find that the standard folders are fine for your needs. Eventually, though, you will probably need folders to help you organize your messages. To create a new folder, select Folder | Create... A window will then prompt you for the name of the folder and allow you to associate or disassociate the folder with certain accounts. If you ever need to change the settings for a folder, select the folder you wish to modify in the Folders Pane and select Folder | Modify... Kmail will then prompt you for the changes you wish to make to the selected folder.
To move messages from one folder into another, select the message(s) you want to move and press the M key or select Message | Move... A list of folders will appear. Select the folder from the list that you want to move the messages to and hit OK. Messages can also be moved by dragging a message from the Headers pane into the Folders pane.
You may find that your folders are taking up a lot of disk space after they have been used for a while even though they may not have very many messages in them. To fix this problem, select the folder and choose Folder | Compact.
If you want to clear all of the messages out of a folder choose Folder | Empty. All the messages in the selected folder will be moved to the trash folder. If the selected folder is the trash folder, all of the messages will be permanently deleted.
Kmail does not currently support hierarchical folders.