How can I check my Mail?
Solution 1:
The mail menu option is hidden in the Social tab in the main menu (Esc on PC).
The fourth tab will include all your mail from manufacturers, NPCs and promotions (Shift-Codes, special editions etc).
Additionally you will be able to send items to your friends in this menu.
Solution 2:
You can directly press F4 (on PC) to access to your mails in-game.
Your mail often contains great weapons which are gift from the companies.