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Deliverability

Hard bounce

A hard bounce is a permanent rejection — the mailbox does not exist, or the domain cannot receive mail. The address should be removed immediately and never retried.

Permanent means permanent. Retrying a hard bounce cannot succeed, and every retry is another data point telling the provider you send to addresses that are not there.

A cluster of hard bounces from one domain usually means the company changed its email convention or shut down, which is a signal about the whole domain rather than one row.

Related terms

Soft bounce
A soft bounce is a temporary rejection — a full mailbox, a server that is down, or a rate limit. The address may work later, so it is retried rather than removed.
Bounce rate
Bounce rate is the share of sent messages the receiving server refused. Under 2% is healthy for cold outreach; above 5% and mailbox providers start treating you as a sender who did not collect their own list.
Email verification
Email verification is checking whether an address can actually receive mail before you send to it. A full check tests the syntax, then the domain's mail records, then asks the receiving server about the specific mailbox.
List hygiene
List hygiene is the habit of removing addresses that bounce, never engage, or should never have been added, so your sending reputation is not spent on people who are not there.