My in-laws have a Mailbox that's a normal looking metal mailbox that's in a brick enclosure. It appears that when it was built, someone just put down mortar, set the mailbox in it, covered the mailbox in mortar and then bricked around it. Fifteen years on the box is rusting and the door has broken off. Any hope of replacing this mailbox somehow or do we just need to tear down the whole brick column and put in something new for a new mailbox?
Is it rusted enough that you can pull the old one out? If so, you might be able to find a similar model that could slot right back in the hole, possibly with a little modification. A plastic one might be a good candidate, both for ease of modification and for weather resistance.
If you do manage to do this, I wouldn't try to mortar the new one in, but instead hold it in with Tap-Cons or similar self-tapping concrete screws. A few of these could be driven into the inside brick surface through the mailbox shell, just to keep things from rattling around too much when you open the door.
Here are a couple links that might help.
If the mailbox is entombed too well you may have to carefully remove some of the bricks until you can get the old box out, install the new mail box, and then replace the bricks you removed.
I have a similar problem, but the door has not broken off. The bottom was rusting out and the exterior was rusting. I used a wire wheel to get the rust off. Then I used Bondo to repair the hole and sanded that. Now after taping we are priming and painting.
The other solution is to cut in out (after measuring it), chip out the mortar until you can slide the new one in then affix it with some screws or caulk and re mortar or use mortar colored sanded caulk at the exterior.