A bounce occurs when an e-mail could not be delivered because of some constant, unchanging condition such as the recipient e-mail address or domain name not existing. When a bounce is logged, the recipient e-mail address is flagged as invalid, and will not be delivered to again in any future deployment, unless the bounce is manually removed. This is to prevent the deliverability issues that arise from resending to bounced e-mail addresses, an act that many ISP's and e-mail providers deem as spam-like behavior.