A "bounce" is a term used to refer to a notice that the email wasn't delivered to the intended recipient. There are many reasons an email could have been "bounced," the main being the email address that was used is no longer valid (e.g. the person deleted the account). Other reasons include an inbox that is full and can't accept any more messages or someone on vacation who has set up an automated vacation message to notify email senders that they are absent.

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