Definition
Bounce rate
The share of sent messages that were rejected and returned, a direct signal to mailbox providers about your list quality.
Bounce rate is the number of messages that came back undelivered divided by the number you sent, usually over a campaign or a day. A bounce means the receiving server refused the message or could not find the address. Hard bounces are permanent (the address does not exist); soft bounces are temporary (full mailbox, server busy).
Mailbox providers read your bounce rate as a statement about how you built your list. A sender who keeps writing to addresses that do not exist is either careless or guessing, and both look like spam. A rising bounce rate therefore does two kinds of damage: the messages that bounced were wasted, and the messages that did not bounce start landing in junk folders as your domain reputation falls. Because reputation is shared per domain, one badly verified list can hurt every mailbox you send from.
The defence is boring and effective: verify addresses before import, treat catch-all results as unknown rather than valid, and stop sending automatically when bounces climb. In Seegnals, verification runs on import and the bounce shield pauses a campaign when bounces cross a threshold you set, then emails you a notification.