If you find yourself in the Historic Center, exploring all the attractions of Mexico City, then you should make a stop at the Palacio Postal.
This actual working post office happens to be an architectural gem. It looks more like a palace than a post office. The interior is spectacular, with stunning staircase, and lavish golden decorations. You can tell that this 19th century building is a fusion of materials and design elements from Europe.
It is worth visiting and admiring the architecture of Palacio Postal, which is why I chose to include it on my list as the 6th must see Mexico City attraction.