Boxer allows users to create custom boxes by taking two or more folders from any of your email accounts and merging them into a new custom view. Creating a custom box is simply creating a new view of your e-mails, not a new folder and it does not delete any of your original folders.
Please take the following steps in order to create a custom box:
1. Tap on the button in the top left corner to access the slide-out navigation pane
2. Choose "Edit" in the right top corner of the slide-out navigation pane
3. Tap the option to "Add custom box"
4. Select the folders from any of your e-mail accounts that you would like to merge and then name your new custom box
5. Tap the arrow in the top left corner
6. Select "Done" in the top right corner to save your new custom box