The folder list in the email view lists all folder that can be used to sort emails into for temporary or final storage. You manage the folders in the settings section and freely choose the hierarchy of the folder tree.
Some of the folders have a special meaning and are used by the system to place messages in. These special folders are indicated with an individual icon in the list and usually cannot be deleted or renamed.
The assignment of special folders can be changed in Preferences > Special Folders.
Filing messages to specific folders is as simple as dragging a message from the list with the mouse and dropping it on the folder you want it moved to. That also works with multiple messages selected.
If the target folder is a subfolder and not currently visible, just hold the mouse over the parent folder while dragging and it’ll expand automatically after two seconds. Move the mouse (while dragging) over the folder list footer to let it scroll until the target folder moves into sight.
There’s also the possibility to copy a message into another folder instead of moving it there. Holding down the Shift key on the keyboard while dropping one or multiple messages on the destination folder will open a menu where you can choose to either Move or Copy them.
When reading a message in the full message view screen, you’ll find a drop-down menu labelled Move to... in the toolbar on top. Select a folder from that menu and the message will be moved there. The message view screen will then load the next message in the list of the current folder.
The selected message(s) can be deleted from the current folder by clicking the Delete icon in the toolbar or by hitting the <Del> (or Backspace on Mac) key on your keybaord. This will by default move the message to the Trash folder and not delete it right away unless you do this for messages in the Trash folder. In order to immediately delete the message in first place, hold down the Shift key on your keyboard while clicking the Delete button or when pressing <Del>.
There’s a preference in Settings > Preferences > Server Settings where the default behavior of message deletion can be changed.