The architecture is beautiful, but there is no mailbox outside the building for after-hours collection. We were staying in the Zocalo area and couldn't find a mailbox anywhere in the vicinity to drop off postcards the night before leaving town. We thought our best bet would be the main post office, but incredibly, no mailboxes on the side or the front of the building. In the end, we used the mailbox inside our hotel, which we were hesitant about because you never know if mail would be collected. Our postcards were postmarked three days after we'd dropped them off, but our friends and family in North America, Europe, and Asia did not receive them until almost 2 months later. The postage we'd paid (over US$1 per card) was for airmail, but it seems the delivery was by surface mail.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.