inbox, in box

Inbox or inbox ( one word ) means a box or tray used to hold incoming mail, messages, or assignments --
My inbox is full, but my outbox is empty. I'd better get to work.

Inbox or inbox also means a computer file that holds incoming messages --
Did you read my email?  Check your inbox.

In box or in box ( two words ) means [ in ] its original container or case ( box ) --
Still new in box; it's a bargain.
The "nib" stands for "new in box".

Now that you know that, you can say --
"There's a message in your inbox, from a guy who wants to sell a classic video game, new in box."