Beautiful museum. I've been to over 50 different countries and I think this gets my vote as the…read moremost beautiful post office (that I've seen) in the world. I think the French colonial post office in Phnom Penh, Cambodia is pretty amazing too, but this is the most gorgeous and intricate (and largest) that I've ever seen. It really is a palace. A great place to start is the actual working post office on the side of the building, which is absolutely gorgeous (and free to visit). Then buy a ticket to the museum, which is wonderful because your entry ticket is an envelope and a real Mexican postage stamp that you can mail; there's a long table inside with pens where you can write a letter and even rubber stamp it with original vintage rubber stamps that they have in the exhibits.
Make sure to take photos from every angle as they're all different. I love walking underneath the staircases because you can see how intricately they are made. Each floor has something amazing. Everything from vintage post office boxes to some incredible paintings of the building, artwork from Mexico City, enormous portraits made entirely from stamps, and vintage Mexican post office uniforms. Some of the offices, which have been kept as they were over 100 years ago, are also beautiful. There is amazing modern art and also classic art. The building itself is just magnificent and the architect is the same one that designed the incredible Palacio de las Bellas Artes.
The staff are genuinely enthusiastic and I felt like they were very happy that people were visiting this gorgeous building. Just don't step too close to the banisters (over the line on the floor) upstairs or the security may start following you. Definitely one of the highlights of my last trip to CDMX.