Email Bounce
What an email bounce is, hard vs soft bounces, how to read codes like 5.1.1 and 4.2.2 and what bounce rate is acceptable. + A table of codes with actions.
An email bounce is a message that the recipient's mail server returns to the sender with a delivery error. A hard bounce is permanent: the address doesn't exist or the server refused for good. A soft bounce is temporary: the mailbox is full or the server is unavailable. In cold outreach, keep your bounce rate below 2%.
At the end of this article you'll find a table of common error codes with a clear action for each: remove the address, retry later or fix your own infrastructure.
At a glance: bounces in numbers
| Fact | Value | Source |
|---|---|---|
| Acceptable bounce rate in cold outreach | below 2%, ideally much lower | Instantly, 2026 |
| Share of invalid addresses after which Mail.ru may send mail to spam or block it | 5% | Mail.ru |
| Permanent failure codes | start with 5 | RFC 3463, 2003 |
| Temporary failure codes | start with 4 | RFC 3463, 2003 |
| Gmail spam complaint threshold (the next risk of a dirty list) | below 0.1%, never 0.3% |
How is a hard bounce different from a soft bounce
A hard bounce won't fix itself: the address has to go. A soft bounce often clears up, and you can resend later.
| Hard bounce | Soft bounce | |
|---|---|---|
| Code | 5.x.x | 4.x.x |
| Typical causes | Address or domain doesn't exist, mailbox disabled, policy rejection | Mailbox full, server overloaded, temporary block, greylisting |
| What to do | Add to the suppression list immediately | Retry later; exclude after 2–3 failed attempts |
| Impact on reputation | Strong: shows your list is dirty | Weak, unless it happens at scale |
How to read a bounce error code
The returned email contains a line like 550 5.1.1 The email account that you tried to reach does not exist. It holds two codes.
- Basic SMTP code (550): three digits, the first is the reply type. 5xx is a rejection, 4xx is a temporary error.
- Enhanced code (5.1.1): class.subject.detail, per RFC 3463. Class 5 means resending the same email won't help. Class 4 means a later attempt may succeed.
The second digit of the enhanced code tells you where the problem is: 1 — the address, 2 — the mailbox, 4 — network and routing, 7 — security and the recipient's policy.
What bounce rate is normal
Below 2% is normal, below 1% is good. Calculate it from emails sent: bounce rate = bounces ÷ emails sent × 100%.
| Bounce rate | Assessment | Action |
|---|---|---|
| below 1% | Good | Keep going |
| 1–2% | Normal | Watch the trend |
| 2–5% | Problem | Pause the segment, run the list through a validator |
| above 5% | Critical | Stop the campaign: Mail.ru may start blocking your email |
Look separately at rejections with code 5.7.x. Their cause isn't your list but your reputation or authentication, so removing addresses won't help.
Why your bounce rate is rising
- An old list. People change jobs, companies change domains, mailboxes get deleted for inactivity.
- A bought or scraped list. Unverified addresses, typos and spam traps.
- Guessed addresses. Patterns like firstname.lastname@company without verification.
- Catch-all domains. The server accepts mail for any address and bounces it later, so a validator can't give a definite answer.
- Problems on your side. Broken SPF or DKIM, a blocklisted domain — these show up as 5.7.x rejections.
How to reduce your bounce rate
- Run the list through a validator before every launch, not once a year.
- Send to catch-all addresses carefully: as a separate segment and at low volume.
- Move hard bounces to the suppression list immediately, so the sequence doesn't send them follow-ups.
- Limit soft-bounce retries to 2–3 attempts.
- Start a new list with a small segment and check bounces before scaling.
Holymailer shows analytics for sent emails and monitors domain blocklists (in Russian), so rising bounces and reputation rejections are visible right away.
Error code table: what to do with each bounce
Find the code in the returned email and follow the action in the last column. Code descriptions follow RFC 3463; individual servers sometimes use them in their own way, so read the error text too.
| Code | Type | Meaning | What to do |
|---|---|---|---|
| 5.1.1 | Hard | The mailbox doesn't exist | Add to suppression list |
| 5.1.2 | Hard | The recipient's domain doesn't exist or doesn't accept mail | Add to suppression list, check for a domain typo |
| 5.2.1 | Hard | The mailbox is disabled and not accepting mail | Add to suppression list |
| 4.2.2 | Soft | The mailbox is full | Retry in 1–2 days; if it repeats, exclude |
| 4.4.7 / 5.4.7 | Soft / hard | Delivery time expired, the server didn't respond for too long | Retry later; if it happens again, exclude |
| 4.7.x | Soft | Temporary policy rejection: rate limiting, greylisting | Lower your sending speed, retry later |
| 5.7.1 | Hard | Delivery not authorized by the recipient's policy or filter | Don't remove the address yet: check blocklists, SPF, DKIM, DMARC |
| 5.7.x mentioning authentication or DMARC | Hard | The email failed SPF, DKIM or DMARC | Fix the sending domain's DNS records |
Frequently asked questions
What is an email bounce in simple terms?
It's an email that didn't reach the recipient and came back with an error. The server's reply contains a code that explains why.
How is a hard bounce different from a soft bounce?
A hard bounce is a permanent error with a 5.x.x code, such as a non-existent address. A soft bounce is a temporary error with a 4.x.x code, such as a full mailbox. Remove the first address; you can write to the second one later.
What bounce rate is acceptable?
Below 2% is the benchmark for cold outreach, according to Instantly. If more than 5% of addresses are invalid, Mail.ru may send your mail to spam or block the campaign.
Should I remove addresses that soft-bounced?
Not right away. Retry in 1–2 days; if the error repeats 2–3 times in a row, exclude the address from the campaign.
Why do I get a 5.7.1 bounce if the address exists?
Code 5.7.1 means a rejection by the recipient's policy, not a missing mailbox. The cause is usually the sender's reputation, a blocklist listing or missing authentication.
How is an email bounce different from a website bounce rate?
They're different metrics with the same English name. In email, bounce rate is the share of undelivered emails; on a website, it's the share of visitors who leave after the first page.
Sources
- RFC 3463: Enhanced Mail System Status Codes — the error code standard, 2003.
- Mail.ru: general mailing rules (in Russian) — the 5% invalid address threshold.
- Instantly: Cold Email Benchmark Report 2026 — data for 2025.
- Google: Email sender guidelines FAQ.
- Holymailer: capabilities (in Russian).