See also: Bounce
CIMM DEFINITION: An e-mail that cannot be delivered to the mailbox provider and is sent back to the e-mail Service Provider that sent it. (Source: IAB)
NOTE – A bounce is classified as either hard or soft. Hard bounces are the failed delivery of e-mail due to a permanent reason, such as a non-existent address. Soft bounces are the failed delivery of e-mail due to a temporary issue, such as a full inbox or an unavailable ISP server. (Source: IAB)