Email bounce
An email rejected by the recipient's server. Permanent when the address does not exist; temporary when the mailbox is full or the server is unavailable.
A permanent bounce — hard bounce — means the address does not exist. You must never send to it again: every additional attempt counts against you with email providers.
A temporary bounce — soft bounce — may resolve on its own. A full mailbox gets emptied, a server comes back online. The address is not therefore condemned.
The two are handled differently, and that is the whole challenge: the first must remove the person from the list, the second must not.
Why it matters
Email providers judge your reputation over time, not on a single send. That is why a dead address costs you long after it was first used: it counts against you in the next campaign, and the one after that.
5 %Above this bounce rate over thirty days, the sending account enters review, then suspension, and deliverability degrades for everything you send — including valid addresses.
Two guards, at two moments — and their thresholds have nothing in common. Over thirty days, the workspace is warned at 3% bounces and 0.08% complaints, and sends are refused at 5% and 0.1%. During an active campaign, a second guard cuts at 25% bounces, after 50 messages sent: this one stops a list where a quarter of the addresses do not exist, before it goes out in full.
The average bounce rate across all industries is 2.5% (WebFX & ClickMinded, 2026). A well-maintained list stays well below that.