You can save messages you received, sent, deleted, saved in any folder.
You can create folders in a folder.

To create folders

On the upper left of the screen,
Click [Create] to create a folder.

Enter the name of the folder and select the parent folder and click [OK].

Move Folder

You can use drag & drop to move the folder you selected to another folder.

Delete Folder

Click the folder you want to delete and click [Delete].

Confirm the message and click [OK].