Borderlands 3 incorporates a mail feature that can be found in other games throughout the generations (especially MMOs). Your Mailbox can be found “in-game” but don’t go looking for a physical mailbox as their isn’t one. For some reason, the developers want you to be logged into a character (any character as your mailbox is universal across all characters).
Once you are in the game with your character, press the Options button, go to the Social option, then navigate over the tabs to the far right to the mail icon. From here you can retrieve any messages the game or other players send you.
From here, press the Square button and your inventory will pop-up. You can then choose an item to send to another player. If you’re just looking to nab this trophy as quick as you can simply send a Level 1 white item picked up off the ground and send to someone on your friends list. It doesn’t matter if they play the game or not, as long as you have one person on your friends list you can net this trophy.