[mk89]

It cannot be a letterbox. You must provide a real address that has capabilities to receive mail.

Luckily for a few euros per month you can rent quite nice addresses and the relative mail forwarding services (e.g., once a day, once a week, etc.)

[niemandhier]

You are right, that what I meant when I said letterbox.

I should have looked up the definition, before using the word.