"Le chat se cachait dans la boîte aux lettres."

Translation: The cat was hiding in the mailbox.

[b_adger]

Duo, many places call it a mail box - not a missing space (example from Australia post at https://auspost.com.au/content/dam/auspost_corp/media/documents/Appendix-02.pdf).

However, mail box and letterbox refer to the box at your house where the post man puts your mail. The box on the street or at the post office is called a "post box" or "posting box".