What is a hard bounce?
A hard bounce is a permanent delivery failure — the address does not exist, the domain does not exist, or the recipient server has permanently refused the mail. Hard bounces are returned with a 5xx SMTP code and should never be retried; repeatedly sending to them is what damages sender reputation.
The distinction that matters is permanence. A hard bounce means the mail will never be delivered no matter how many times you try, and mailbox providers treat continued attempts as a signal that you are not maintaining your list — which is exactly the signal that moves your mail to spam.
Common causes, in rough order of frequency: the mailbox was closed when someone left a company, the address was mistyped at signup, the domain expired, or the address never existed because it was invented to get past a form.
A bounce rate above roughly 2% is the point at which most mailbox providers start throttling. Verification exists to get that number down before you send, because after you send the damage is already recorded against your domain.
Last reviewed 2026-09-11