The mailbox is one of the possibilities in the Bus Company Simulator to communicate with other players. Each player has his own private mailbox and every company has additionally a central company mailbox. Every company can decide which employees should have access to the company mailbox.
When you write a new message, you can choose to send the message to a company, to a specific person of the company (see screenshot) or to any player (username). After selecting the recipient, the language of the recipient is displayed in the form of a flag.