What is Hard bounce?

A hard bounce is a permanent delivery failure: the receiving server says the address or domain does not exist and will never accept the message.

Also called: permanent bounce, soft bounce.

The receiving server signals a hard bounce with a permanent (5xx) SMTP response, typically meaning the mailbox is unknown, the domain has no mail server, or the domain itself does not exist. Retrying will not help, so a sending tool should stop mailing that address after the first hard bounce.

A soft bounce is the temporary (4xx) kind: a full mailbox, a server that is briefly unavailable, or rate limiting. Tools usually retry soft bounces for a while and convert them to hard bounces if they keep failing.

Hard bounces are what SMTP verification predicts. An address the server refuses during verification is the address that would hard-bounce in a campaign, which is why verification before sending lowers the bounce rate and protects sender reputation.

Every hard bounce belongs on your suppression list so it is never mailed again, from any campaign or any tool. When you import a new list, compare it against past bounces before sending.