Mailboxes are usually the jurisdiction of the federal government - post office. Often postal carriers are the best people to speak with regarding this. I would try to catch your mail carrier when they enter and explain the situation or leave a note for them to call you. And if they are doing you a favor, remember to be kind to the postal workers around Christmas time. However, one thing to note is that with an outside post box, you will not be able to get the bulk mail stuff that you may or may not want. Flyers, phone books, newspapers, coupons. Anything that is not specifically addressed to a unit will likely not be delivered. That could be a good thing too. One more consideration, you will have to have individual mailboxes for each unit as the post office must deliver to individuals, not a whole building. Based on the area, the city may have a say in what these outside boxes look like. If you pattern after ones in the area, you may be ok. I would ask the postal carrier first and then maybe talk to the city about aesthetics.