What Is Email Bounce Rate? (Hard vs Soft)
Email bounce rate is the percentage of your sent emails that could not be delivered and were returned by the receiving mail server. You calculate it as bounced emails divided by total emails sent, times 100. Bounces split into two types: hard bounces, which are permanent failures like a nonexistent address, and soft bounces, which are temporary failures like a full mailbox.
How to calculate bounce rate
The formula is straightforward, but the denominator trips people up. Use emails sent, not emails delivered — dividing by delivered emails hides the problem you're trying to measure.
Bounce rate = (bounced emails ÷ emails sent) × 100
If you send 10,000 emails and 240 bounce, your bounce rate is 2.4%. Most platforms also report hard and soft bounce rates separately, and that split is where the actionable information lives. A 2.4% bounce rate that's almost entirely soft bounces from one temporarily overloaded corporate server is a very different situation from 2.4% hard bounces spread across your list.
Hard bounces vs soft bounces
A hard bounce is a permanent delivery failure. The address doesn't exist, the domain doesn't exist, or the recipient's server has flatly refused you. These addresses will never accept mail and must be removed immediately — most reputable sending platforms suppress them automatically after a single occurrence.
A soft bounce is a temporary failure. The mailbox is full, the server is down, the message exceeded a size limit, or the receiving system is throttling you. The address may work fine tomorrow. The standard practice is to retry for a set window, and to convert an address to a hard bounce after it soft bounces on several consecutive sends.
| Attribute | Hard bounce | Soft bounce |
|---|---|---|
| Nature | Permanent failure | Temporary failure |
| Typical SMTP code | 5xx | 4xx |
| Common causes | Address doesn't exist, domain doesn't exist, typo, blocked sender | Mailbox full, server down, message too large, greylisting, rate limiting |
| Retry? | No | Yes, usually 24–72 hours |
| Action | Remove from list immediately | Monitor; remove after 3–5 consecutive soft bounces |
| Reputation impact | Severe | Mild unless persistent |
What the SMTP codes actually mean
Your bounce logs contain the receiving server's own explanation. Learning to read the first digit tells you most of what you need to know: 4xx is "try again later," 5xx is "don't try again."
| Code | Meaning | Type | What to do |
|---|---|---|---|
| 421 | Service not available, try later | Soft | Retry; check if you're being throttled |
| 450 | Mailbox unavailable (temporary) | Soft | Retry; often greylisting |
| 452 | Insufficient storage — mailbox full | Soft | Retry; drop after repeated failures |
| 550 | Mailbox unavailable / user unknown | Hard | Remove immediately |
| 551 | User not local | Hard | Remove |
| 552 | Storage allocation exceeded | Soft/Hard | Read the text; usually retry once |
| 553 | Mailbox name not allowed / invalid syntax | Hard | Remove; often a typo |
| 554 | Transaction failed — often blocked as spam | Hard | Investigate reputation and authentication |
A 554 deserves special attention. It frequently means the receiving server accepted the connection but rejected the message on reputation or content grounds, which is a warning sign about your sending setup rather than the recipient's address.
What counts as a good bounce rate
As a working benchmark, keep your total bounce rate under roughly 2%, and your hard bounce rate under about 0.5%. Anything above that range should trigger an investigation before your next send. Mailbox providers watch bounce rates closely because a high rate is the clearest signal of a list acquired rather than earned — nobody who collected addresses through a legitimate signup form ends up mailing thousands of nonexistent mailboxes.
Context matters. A first send to a list that's been dormant for two years will bounce far more than a weekly newsletter to engaged subscribers, and that's expected. What isn't acceptable is a bounce rate that stays elevated send after send, which means you're not processing bounces properly.
Why emails bounce
- Typos at signup. "gmial.com," "yaho.com," and missing letters in the local part account for a large share of hard bounces on otherwise legitimate lists.
- Employee turnover. B2B lists decay fast. Business addresses go dead when people change jobs, and a list you built two years ago may have lost a quarter of its corporate addresses.
- Purchased or scraped lists. These are dense with dead addresses and spam traps. They are the single fastest way to destroy a sending domain.
- Spam traps and role accounts. Addresses like info@, sales@, and abuse@ often bounce or generate complaints, and recycled spam traps look like ordinary addresses until they wreck your reputation.
- Missing authentication. Without correct SPF, DKIM, and DMARC records, some receiving servers reject outright rather than filter to spam, which surfaces as bounces.
- Blocklisting. If your IP or domain lands on a blocklist, entire providers may reject your mail with 5xx codes even though every address is valid.
- Oversized messages. Heavy image attachments can exceed recipient limits, particularly on corporate mail systems with conservative caps.
How to lower your bounce rate
- Verify your list before big sends. Running addresses through an email verifier catches syntax errors, dead domains, and known-invalid mailboxes before they ever hit a mail server. This is the highest-leverage single action on this list.
- Use confirmed opt-in for new subscribers. A confirmation click proves the address exists and is monitored, eliminating typos at the source.
- Add real-time validation on your signup form. Catching "gmial.com" while the user is still on the page beats discovering it three weeks later.
- Authenticate your domain. Correct SPF, DKIM, and DMARC records prevent an entire category of rejection. Modern platforms set these up automatically when you connect a sending domain.
- Suppress hard bounces automatically. One hard bounce, permanent removal. Never re-import a list that includes previously bounced addresses.
- Set a soft-bounce threshold. Three to five consecutive soft bounces should promote an address to suppressed.
- Re-engage or remove dormant subscribers. Segments that haven't opened in a year are where dead addresses and recycled spam traps concentrate.
- Warm up new domains gradually. Sending 50,000 emails on day one from a brand-new domain invites blocks that show up as bounces.
- Send consistently. Long gaps between campaigns let list decay accumulate and reset any reputation you've built.
Bounce rate is really a symptom, not a disease. It's one of several signals — alongside complaint rate, spam-folder placement, and engagement — that together describe whether mailbox providers trust you. Treating it as part of a broader email deliverability strategy gets you further than chasing the number in isolation.
What to do after a high-bounce campaign
- Pause further sends to that list segment immediately.
- Export the bounce log and separate hard from soft.
- Read the actual SMTP response text — it usually names the reason in plain English.
- If you see 554s or blocklist references, check your domain and IP against major blocklists.
- Suppress every hard bounce before you send again.
- Verify the remaining list, then resume at reduced volume to your most engaged segment first to rebuild positive signal.
Frequently asked questions
What is a good email bounce rate?
Under roughly 2% total bounce rate is a reasonable target, with hard bounces below about 0.5%. Rates consistently above that range suggest list quality or authentication problems that will start affecting inbox placement.
What is the difference between a hard bounce and a soft bounce?
A hard bounce is a permanent failure — the address or domain doesn't exist — and the address should be removed immediately. A soft bounce is a temporary failure like a full mailbox or an overloaded server, and it's worth retrying for a few days before giving up on the address.
Do bounced emails hurt my sender reputation?
Yes, particularly hard bounces. Mailbox providers treat a high rate of mail sent to nonexistent addresses as a strong signal of a purchased or poorly maintained list, and that judgment affects whether your future messages reach the inbox or the spam folder.
Should I resend to an address that soft bounced?
Yes, but with limits. Most platforms retry automatically over 24–72 hours. If an address soft bounces on three to five consecutive campaigns, treat it as permanently undeliverable and suppress it.
How do I find out why my emails are bouncing?
Check your platform's bounce log for the SMTP response code and message text returned by the receiving server. Codes beginning with 4 are temporary, codes beginning with 5 are permanent, and the accompanying text usually states the specific reason.
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