Does κουτί mainly refer to a mailbox like in the photograph, or can it be used to describe something like a cardboard box?
κουτί can be box, carton, crate even a can/tin etc. "mailbox" in Greek is "γραμματοκιβώτιο" γράμμα (letter both ABCs and one that you mail) + κιβώτιο (a box but with more of a technical usage. "gear box" κιβώτιο ταχυτήτων). So, "box" is "κουτί" for cookies, or tissues and "κιβώτιο" is for more important things.
Wow, that was quite thorough, thank you! The photograph illustrating the word was a mailbox, which made me confused. Also, it is interesting to see that letter is a homograph in Greek too.
It literally means something like "something written" - result noun from γράφω "to write".