Virtual inboxes:
Desktop, Documents, Downloads.
Buffer files to write elsewhere.
Notes for clients, old notes.
Anything anywhere to be processed.
All emails, incoming or archived.
Reader, pinboard, etc.
Real inboxes:
Desktop, paper tray, paper slips box.
Pocket briefcase, index cards box, pinboard.
Notebooks, a backpack full of them, closets.
The mailbox, other people, the answering machine.
We have many boxes.