The 'ufficio inesitati' is where a parcel/letter is taken to if it requires signing for, and you are not in at the time of delivery. Monolingual dictionary defines it as: Detto di pacco o di corrispondenza che le poste non hanno fatto giungere a destinazione.

I don't know if there is an official term in English, but I'd probably just put sth like: Should delivery be to the relevant post office (and not to the default post office where delivery fails/is impossible)....

Hope this helps! :-)