What it means
Bounce rate is the share of sent emails that could not be delivered. A hard bounce is permanent: the address does not exist, or the domain is gone. A soft bounce is temporary, such as a full mailbox, a server outage or a message that is too large.
Hard bounces are the ones that hurt. Mailbox providers read a high bounce rate as a sign the sender is mailing a list nobody checked. They then filter that sender's future emails more strictly, including the ones sent to valid addresses.
How we use it
We aim to keep bounces off a client's domains entirely. Every address is verified before it enters a sequence, and catch-all addresses get a second check. A list that has sat for months is checked again before it goes back into a campaign.
When bounces do appear on a campaign, we look at the list first. Our List Diagnostic skill starts with that question.