Mail Bounce
When sending E-Mails the client or the server may define a Return-Path header. This tells the receiving mail server in case the mail cannot be delivered, where to return the mail.
A mail can be bounced hard and soft. A hard bounce means f.g. the recipient does not exist and do not try to redeliver the mail. A soft bounce means f.g. the mailbox of the recipient is full and should try to redeliver the mail.