We haven't had post offices for over a decade. The one in my home town was a modern construction and made place for housing. In larger towns and cities, there were monumental post office buildings. Some have been refurbished.
Amsterdam's main post office was built between 1895 and 1899. It stands behind the Royal Palace that stands on Dam Square. It was refurbished and has become a shopping mall named Magna Plaza.
https://en.wikipedia.org/wiki/Magna_Plaza