Folders appear to the left side of the mailbox. There are a few folders that appear by default, and these can not be edited. You can create as many or as few folders as you like, nest folders into subfolders, or create folders based on search criteria.
If you do not see the folder list, you should see a hide/show button in the top-left of the page. Click the button to show the folder list. To set the window so the folder list always shows, go to the Settings → Theme screen.
At the bottom of the folder list, there's a blank folder. Type a name for the folder you want to create in the field next to the folder icon.
You can drag and drop folders around in the list to reorder them. Or, drag a folder on top of another to nest them.
FastMail allows you to save your common searches for quick access in the folder list. To do this:
In the folder list, click the Save button next to your search results. A naming box appears.
Name your search and press the Save button. The search now appears as a folder in the folder list.
Please note, saved searches are only visible in the web interface. It is not possible to access them via external (IMAP) clients.