The post office is an extremely beautiful building on the exterior and interior.
Most tourists will visit Palacio de Bellas Artes and it's worth a trip to the post office while you're doing that as well.
It's a block or two away and you will only need 15-20 minutes here.
You can walk around the lower level which is an actual post office that is in use and then take some pictures on the staircase.
You can't go upstairs.
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