Here's a post that might help you - https://www.diynetwork.com/how-to/outdoors/structures/building-a-stone-mailbox
Mailbox or mailbox post? Also check postal regulations. They specify what you can and cannot do and distance for height and edge of road.
First you need to get the regulations from your areas post office. Also one thing to consider if enclosing a mailbox with brick get a few. My friends did that and mailbox door broke off and rusted out. Could not find that exact one had to tear down. And rebuild. Very expensive. They bought 3 mailboxes as replacements when and if needed.
I was a mail carrier for 30 years ! The worst mailboxes were the ones set in stone or brick !!!! The doors always break off after awhile , leaving the mail in the box exposed to the elements and theft . Plus it is an eyesore . The only way to get around this issue is to have a good quality metal mailbox installed with the door hinges exposed past the stone or brick face of the structure . If you can't do this , don't bother building one !
Check with your post office to see what the regulations are before you do anything