If the word "Inbox" is not listed in the Spell dictionary then why is it spelt that way in my Inbox?

Interesting i quite often often have yahooing and googling come up as non words.

In box spelt as inbox is recognised by all but originally it would of been spelt as 2 words not 1.If the word "Inbox" is not listed in the Spell dictionary then why is it spelt that way in my Inbox?
It's a relatively new word. Languages change and evolve over time, and that includes the addition of new words. The word "inbox" only came into being with the rise of email communication in the 1990s, so it hasn't been around long enough to be in every dictionary yet.If the word "Inbox" is not listed in the Spell dictionary then why is it spelt that way in my Inbox?
because it is