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Deliverability

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.

Most sending platforms retry automatically for a day or two and then give up. The judgement call is what to do after that: an address that soft-bounces on every send across several campaigns is functionally dead even though no server ever said so, and keeping it costs volume for nothing.

A practical rule is to suppress after three consecutive soft bounces on separate days, which catches abandoned mailboxes without discarding someone who was on holiday with a full inbox.

Related terms

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.
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.
Greylisting
Greylisting is a spam defence where a server temporarily rejects mail from an unfamiliar sender and accepts it on the retry. Real mail servers retry; most spam software does not.
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.