Say you send 1,000 emails and 80 come back undelivered. Money spent, subscribers missed, and your sending platform flags the account for review.
That returned email is a bounce. The share of bounces out of total sent is your bounce rate. It sounds like a minor stat, but ignoring it leads to a predictable chain: spam folder, then account suspension.
This article covers the full picture: what bounce rate measures, the difference between bounce types, what those three-digit SMTP codes actually say, and the specific steps that bring the number down.
What is email bounce rate, in plain terms
Think of physical mail. You address an envelope, drop it in the post — and a week later it comes back stamped "addressee not found." The building was demolished. Or the street number was wrong. Or the mailbox was stuffed full.
Email works the same way. Bounce rate is the percentage of messages that did not reach the inbox and were returned to the sender.
The formula
Bounce rate = (bounces ÷ sent) × 100%
Send 1,000 emails, get 30 back → bounce rate = 3%. That puts you in the yellow zone.
Every major sending platform — Mailchimp, SendGrid, Brevo — reports bounce rate in campaign stats. Check it after every send, not just when something feels wrong.
Hard bounce vs soft bounce: what makes them different
Not all bounces are the same problem. Some are permanent, some are temporary — and the right response depends on which you are looking at.
Hard bounce
A permanent failure. That email will never be delivered.
🏚️ Analogy: the building was torn down — the address no longer exists.
Common causes
- • Address does not exist (deleted or made up)
- • Domain is not operational
- • Receiving server has permanently blocked the sender
⚡ Action: remove from your list immediately
Soft bounce
A temporary failure. Delivery might succeed on a retry.
📬 Analogy: the mailbox is jammed — it exists, just can't take anything right now.
Common causes
- • Mailbox is full
- • Receiving server is temporarily down
- • Message size exceeds the server limit
⚡ Action: retry 2–3 times, then remove if still failing
Two less common types: General bounce — server rejected without explanation. Auto-reply bounce — the recipient's out-of-office responder. Not a real failure, but it pollutes your stats.
SMTP error codes: what the receiving server is telling you
When a message bounces, the receiving server attaches a three-digit code to the rejection. Read it the way you would read a diagnostic code — it tells you what broke and whether it can be fixed.
The codes you will see most often in campaign reports:
Mailbox unavailable / User unknown
Hard bounceThe address does not exist. Most common bounce code. Deleted mailbox, typo, or a domain that never hosted email.
User not local
Hard bounceThe server does not handle this address. The message landed at the wrong host.
Mailbox name not allowed
Hard bounceThe address format is invalid — disallowed characters or a malformed local part.
Exceeded storage allocation
Soft bounceMailbox is full. The address is real, but there is no room for new messages.
Service not available
Soft bounceServer is temporarily unavailable. Try again later.
Mailbox busy
Soft bounceMailbox is temporarily locked. The server is handling other requests.
Local error in processing
Soft bounceAn error on the receiving server's side. Not your problem to fix.
Insufficient system storage
Soft bounceThe receiving server has run out of disk space. A temporary condition — retry later.
Transaction failed
BlockGeneric rejection, often from a spam filter. The server decided the message looked suspicious.
Code 550 accounts for the majority of hard bounces. It is also the most expensive: you paid to send to an address that cannot receive anything.
What bounce rate is actually acceptable
Lower is always better. Here are the thresholds that matter:
< 2%
Green zone
List is clean, domain reputation is healthy. Keep doing what you are doing.
2–5%
Yellow zone
Time to act. Clean the list and validate addresses before the next send. Not catastrophic yet, but heading in the wrong direction.
> 5%
Red zone
Critical. Real risk of account suspension and all messages routing to spam.
Mailchimp's industry data put average hard bounce rates below 1.3% across verticals. Construction sits near the top at 1.28%; coupon sites near the bottom at 0.13%.
MailerLite analyzed roughly a million campaigns and found a similar range: 0.2% for publishers, up to 1.32% for architecture and construction.
If you are above 2%, your list is dirtier than the industry average. That is where to start.
7 reasons your bounce rate keeps climbing
1.Typos in addresses
gmial.com instead of gmail.com. People mistype at signup and the error lands in your list. One bad character, one dead address.
2.Purchased lists
Buy a list of 100,000 addresses and you are likely looking at 40–50% dead, plus spam traps throughout. The survivors do not know you. Result: hard bounces, abuse reports, suspension.
3.No double opt-in
Without email confirmation at signup, typos, bots, and placeholder addresses all get through. Double opt-in filters them before they touch your sending reputation.
4.List that has not been cleaned recently
Addresses go stale. People leave jobs, abandon old inboxes, or switch providers. A list clean 18 months ago may have 5–10% dead addresses today. Validate every 3–6 months.
5.Missing SPF, DKIM, or DMARC
Without these DNS records, your messages arrive looking unauthenticated. Some servers reject them outright, which shows as a bounce even though the address is valid.
6.Spammy content or subject lines
"FREE!!!", "You have been selected", "Act now." Spam filters catch these patterns and block delivery before the message reaches the inbox. SMTP code 554 is often the result.
7.IP or domain on a blocklist
If your sending IP ended up on a blocklist, receiving servers will reject your mail. Check mxtoolbox.com to see if you are listed.
How to reduce your bounce rate: a 7-step checklist
In most cases you can get bounce rate under control in a single day. Here are the seven steps, ordered by impact.
Validate your list before the next send
This is the fastest lever. Upload your list to a validator, let it identify dead addresses, catch-all domains, and disposable inboxes, then remove them before hitting send. Bounce rate drops immediately.
Upload your list to uChecker and get a breakdown of invalid, non-existent, and risky addresses in minutes. Remove them before you send. 30 free checks to get started.
Validate your list for free →Turn on double opt-in
Subscriber enters an address, receives a confirmation email, clicks the link. If the address is wrong, the confirmation never arrives. Typos and bots are filtered before they enter your list.
Add real-time validation to signup forms
Check format as the user types. No @ symbol? Domain missing a dot? Show the error on the spot rather than discovering it in your next bounce report.
Clean your list on a schedule
Run a validation pass every 3–6 months. Remove subscribers who have not opened anything in 90 days or more. Addresses die constantly, and there is no way around it.
Set up SPF, DKIM, and DMARC
Three DNS records that tell receiving servers your mail is legitimate. Without them, more messages get rejected. Most hosting providers walk you through setup in under ten minutes.
Check bounce rate after every campaign
Do not just send and move on. After each campaign, note how many bounced and what type. A sudden spike means something changed — catch it early before it compounds.
Never buy lists
Only collect addresses through forms, lead magnets, webinars — places where people actively opted in. A purchased list is a guaranteed path to hard bounces, spam traps, and abuse complaints.
What happens when you ignore bounce rate
Nothing good. The chain of events looks like this:
Bounce rate rises as more messages fail to deliver
Mailbox providers lower your domain reputation based on the failure signals
Messages start routing to spam, even for subscribers with valid addresses
Your ESP suspends or terminates your sending account
Campaign spend is wasted, and the audience you built erodes
Rebuilding domain reputation after a suspension is slow and painful. Routine list validation is far cheaper than the recovery.
The short version
Bounce rate is one of the clearest signals of list health. Keep three things in mind:
- ●Hard bounce — address is dead, remove it immediately
- ●Soft bounce — temporary issue, monitor and retry
- ●Target — stay below 2%; above that, clean the list
Start with the fastest fix: validate your current list. It takes a few minutes and immediately shows you where the dead weight is.
Validate your list in uChecker — 30 free checks to see the scope of the problem.
Validate your list for free