Inbox isn't a word?

According to the spell checker of thunderbird, inbox is not a word. They are not the only one too. Openoffice doesn't recognize it. Inbox is used in almost every single email program that exists. Its been around for at least 15 years and yet its not consider a word. These spell checkers will recommend in box. D'oh is in Webster which it shouldn't be but inbox isn't. No wonder people won't accept mindframe as a word