Hard bounces
A permanent failure. The address does not exist, the domain does not resolve, or the server has refused you outright. These come back with 5xx SMTP codes and they will never succeed no matter how many times you retry.
Hard bounces must be removed from your list immediately. Retrying them is one of the clearest signals to a receiver that you are not managing your list, and abandoned addresses that keep receiving mail are precisely what get converted into recycled spam traps.
Soft bounces
A temporary failure — mailbox full, server down, rate limited, greylisted. These return 4xx codes and mean try again later. Sending servers retry automatically on a backoff schedule, usually for a couple of days.
Soft bounces are not immediately damaging, but a persistent one is a hard bounce wearing a disguise. A rule that converts an address to hard after a handful of consecutive soft failures is standard practice.
The threshold that matters
Keep hard bounce rate under two percent. Above that, receivers begin treating your sending as careless and placement suffers across the whole domain. Above five percent you should stop sending and fix the list before doing anything else.
Cold email lives or dies on this, because cold lists are inherently less accurate than opt-in ones. Validation before sending is not optional at volume — it is the single highest-leverage piece of list hygiene there is.
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Common questions
What bounce rate is acceptable?
Under two percent hard bounces. Between two and five is a warning. Above five, stop and clean the list.
Does my sending tool remove bounces automatically?
Most do for hard bounces within a campaign, but not always across campaigns or workspaces. An address that hard bounced in one sequence can easily be mailed again from another. Deduplicate and suppress at the list level, not per campaign.
What is greylisting?
A receiver deliberately soft-bouncing a first-time sender to see whether they retry properly. Legitimate servers retry and get through; much spam software does not. It is a normal delay, not a problem.
Related concepts
If this is the thing going wrong
The pages explain it. If you would rather it was simply fixed, that is the work I do.