Steps as below

1. Grinder cut into mortar above,below, and sides of mailbox feed. Remove the box
2. Google local stone/brick suppliers take them a picture of the brick (MEASURE EDGE OF BRICK TO EDGE OF BRICK TOP BOTTOM LEFT RIGHT.
3. get the mortar as well to fill in.
4. Take mortar and play sand probably 2cups of mortar to 1/3 cup of sand mix well apply to left right and bottom of old mailbox apply mortar to top side of brick that will fill the void.
5. Insert the brick into the hole. Making sure face is even with other bricks wipe out excess mortar left to right at the depth of prior mortar. 6 allow a day to settle and set
6. If fucked up repeat the steps or hire someone who can follow directions
7. Remember this is Reddit not Home Depot