Where is the apartment mailbox hidden?!

You do not have to download a MOD etc. All you have to do is go into a premade building that has an apartment mailbox. Then, instead of trying to Copy the mailbox by using the eyedropper tool, use the Create tool, which allows you to change the color of the object etc. Then once you create a new custom color, Upload your creation. Exit out of the game, upload it and then download it into your game. Once it is downloaded, do the "testingcheatsenabled on" cheat, then do the "buydebug" cheat and finally do the "restrictbuildbuyinbuildingscheat off" and FINALLY- once you click on the buydebug question mark in buymode, scroll down and you will see the mailbox as an option. You can add it to your custom made building. Sounds complicated but it is not. Follow all the upload steps as you would if you were uploading a family or an object. I prefer this method instead of downloading MODS or content from unknown sims sources. Try it, it works! It worked fine for me.