Complex sentence: Larger cities/towns have what they call "Pochtamt"-Main Post Office, which has the main postal code for the city, for instance Moscow's Pochtamt has a postal code 101000.
Split into simpler sentences: Larger cities/towns have what they call "Pochtamt"-Main Post Office, which has the main postal code for the city. For instance Moscow's Pochtamt has a postal code 101000.