Hard and soft, and why the split matters
A bounce is a rejection by the receiving server. A hard bounce is permanent, typically a 5xx response meaning the address does not exist. A soft bounce is temporary, a 4xx response such as a full mailbox or a rate limit, and the same address may accept mail later.
The distinction is not academic, because the two carry completely different signals. Soft bounces are mostly noise about the recipient's mailbox state. Hard bounces are a statement about your list: you are sending to addresses that were never real, or have not been real for a long time. That is what moves reputation.
What the numbers look like in practice
Concrete example from our own data. A single campaign we ran through Brevo produced 8,730 addresses to suppress: 3,708 hard bounces, 3,904 soft bounces and 1,105 unsubscribes. That is the shape of a list that had not been cleaned, and every one of those hard bounces was a small deposit against the sending reputation.
Another: one customer account, Obooko, showed a seven-day bounce rate of 3.46% and climbing, with 3,350 hard bounces already accumulated. We have also seen an account-wide bounce rate of 3.52% inside a single 24-hour window. Those are the numbers at which intervention is not optional, because the trend matters more than the level. A stable 3% is a bad list. A 3% climbing is an ongoing event.
Bounce rate, read honestly
7-day rate, and what it means for a normal listSuppression is the actual control
The suppression list, not the send log, is where deliverability is really managed. We count soft bounces per email channel and suppress a recipient automatically once they hit the threshold, which for us is three consecutive soft bounces. Hard bounces are suppressed on the first occurrence, because there is no version of the story where the address becomes real again.
This is not a new lesson. SmartrMail had the same mechanism, automatically excluding recipients after repeated soft bounces, and the reason it exists in both products is that manual list hygiene does not happen. It has to be enforced by the system or it does not get done.
The scale point is worth stating plainly: on one of my own flows, a customer onboarding sequence running to 596,000 subscribers, suppression is the only thing standing between a normal send and a reputation incident. At that size, a 1% difference in bounce handling is 6,000 messages into a wall.
Preventing them
Most hard bounces are preventable at capture. Validate addresses at the point of collection rather than at send time, use confirmed opt-in where the list will be long-lived, and re-verify segments that have gone quiet for a year or more.
After that it is discipline: never import a purchased list, never resurrect an old export because a campaign needs volume, and let the suppression list do its job rather than clearing it to inflate reach. If bounce rate has already damaged placement, fix the list first, then confirm authentication is aligned, check you are not listed, and measure real inbox placement before deciding whether the problem is solved.
Catching bad addresses before they bounce is what the email validation API is for, and bounce events arrive in real time on the webhooks feed so suppression can happen without a nightly job.
Go deeper
References
The primary sources behind the rules on this page. Provider policy and the underlying standards, not vendor marketing.
Common questions
Under 2% is healthy and under 1% is good. Above roughly 3% and climbing is the point at which reputation damage becomes measurable.
A hard bounce is a permanent 5xx rejection, such as an address that does not exist. A soft bounce is a temporary 4xx condition like a full mailbox, and may succeed on retry.
We suppress a recipient on a channel after three consecutive soft bounces. A hard bounce is suppressed on the first occurrence.