Email bounce rate
Email bounce rate is the percentage of emails you sent that the receiving server refused or returned, worked out as bounces divided by emails sent.
Also called Bounce rate, Hard bounce, Soft bounce.
Email bounce rate is the share of your sent email that came back. The formula is bounces divided by emails sent, times a hundred: send 200 emails and get 6 back, and your bounce rate is 3%. It is the first number a mailbox provider looks at when deciding what kind of sender you are, because it is the one number a sender cannot argue with. A message either reached a mailbox or it did not.
A high rate says something specific about you. It says you are writing to addresses you have not checked, which is what a bought or scraped list looks like from the receiving end, and providers respond by filtering the rest of your mail before anyone reads it. The bounces themselves cost nothing. The reputation they cost the sending domain takes weeks to recover.
Hard bounce vs soft bounce
A hard bounce is permanent. The address does not exist, the domain has no mail server, or the receiving server has refused your mail outright. Nothing you do will get a message through, and sending again only repeats the damage, so a hard-bounced address goes straight onto the suppression list.
A soft bounce is temporary. The mailbox was full or the message was too large, the receiving server was busy or briefly unreachable, or greylisting asked the sender to try again later. Sending systems usually retry a soft bounce on their own for a day or two. An address that soft bounces repeatedly is usually a hard bounce that has not admitted it yet, and it is reasonable to stop after the second or third.
What is a good bounce rate
The figure most often quoted is 2%, and it is a rule of thumb rather than a published threshold: mailbox providers do not say where their line is, and it moves with everything else they know about you. What can be said is that a verified list of people who actually work where you think they work bounces well under that, and that a rate climbing past a few percent is a list problem rather than bad luck.
Keeping it low is mostly done before the first send. Verify every address, so the ones that do not exist never leave the building. Stop after a hard bounce and never try that address again. Ramp a new domain slowly, because a bounce on a domain with no history counts for more than the same bounce on one with years of clean sending behind it. And treat a rising rate as a signal to stop and look at the list, since sending through it makes the next campaign worse.
How Kamu handles bounces
Kamu checks an address before the first email to it, and an address that fails verification is marked as bounced without a message ever being sent. Delivery is recorded per email from the sending provider's own events, so "it never arrived" and "nobody replied" appear as different lines. A bounce breaker watches the last two weeks of sending: once at least twenty emails have gone out and 5% or more of them have bounced, sending for that product stops while the rate stays there.
In practice
- 200 emails sent, 6 returned as undeliverable: a 3% bounce rate.
- A soft bounce from a full mailbox on Friday that delivers on the retry on Monday.
- A list exported from a CRM last touched two years ago, where many of the addresses belong to people who have since left.
Questions
What is a good email bounce rate?
Under 2% is the figure most people repeat, and it is a rule of thumb rather than a number any mailbox provider has published. For cold email the honest target is as close to zero as verification can get you, because every hard bounce is an address that could have been checked in advance. A rate that is climbing matters more than any absolute level: it means the list is decaying or was never verified, and the fix is to stop and clean it rather than to keep sending.
What causes a soft bounce?
A temporary condition at the receiving end: a full mailbox, a message over the size limit, a server that was down or busy, or greylisting that asks unfamiliar senders to retry. Sending systems usually retry on their own for a day or so. If the same address soft bounces on several attempts it is behaving like a hard bounce and should be treated as one.
Do bounces hurt my sender reputation?
Yes, and faster than almost anything else. A hard bounce tells the receiving provider you are mailing addresses you did not check, which is what a purchased list looks like from their side. Providers weight it heavily because it is hard to fake and easy to avoid. A run of bounces on a young domain can get the rest of its mail filtered for weeks.
Should I remove soft bounces from my list?
Not after one. A full mailbox is not a dead address, and most systems will retry and deliver. After two or three soft bounces to the same address across separate sends, treat it as hard: something at that mailbox is not going to change, and each further attempt adds to your rate without any chance of a reply.
Related terms
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