Ding ding ding ding, you have an email. Problem: You don't know how to open the Borderlands 3 mailbox. Logically, the latter should be in the menu of your character and your inventory, but nope: your precious messages filled with Legendary Weapons and Golden Keys are in another castle.
In and out of combat, press Esc, or start according to your support, then look for the social tab. Looking in the left pane, you can admire the email icon leading to all your messages and rewards piled up since your first launch of the game. It is also where Hammerlocke will send your hunting rewards, which makes the choice to have stuck it with game options even stranger.