You’ve probably seen a “delivery failed” reply land in your inbox. But what does it actually mean when an email bounces?
And how do you tell a bounce you can ignore from one you need to fix today?
That’s the split between a soft bounce and a hard bounce. Getting it right protects your sender reputation. Getting it wrong can put your domain on a blacklist.
In this guide, we’ll explain what a bounce is and how to read the error code behind it. Then we’ll cover how to fix both kinds.
- What Is a Bounced Email?
- Hard Bounce vs. Soft Bounce Emails
- How to Read Your Bounce Message
- Why Bounce Types Aren’t Always Clear-Cut
- How to Fix a Bounced Email
- What’s a Good Email Bounce Rate?
- How to Check Which Emails Bounced
- How to Improve Your Email Bounce Rate
- How SendLayer Helps You Manage Bounces
- Frequently Asked Questions
What Is a Bounced Email?
A bounced email is one the receiving server rejects. Instead of reaching the inbox, it “bounces” back to you.
That bounce isn’t just a rejection. It’s a report. The receiving server sends back a non-delivery receipt, and it usually tells you exactly what went wrong.
A bounce message normally includes three useful things:
- The name of the server that rejected your email
- The date and time it was rejected
- A numeric status code explaining why
The status code is the part worth learning. It’s the difference between guessing and knowing.
Hard Bounce vs. Soft Bounce Emails
There are two types of bounces: hard and soft. Here’s the short version before we get into detail.
| Soft bounce | Hard bounce | |
|---|---|---|
| What it means | Temporary delivery problem | Permanent delivery problem |
| Typical code class | 4.X.X | 5.X.X |
| Address valid? | Usually yes | Usually no |
| Will it retry? | Yes, automatically | No |
| Do you need to act? | Only if it repeats | Yes, right away |
| Effect on reputation | Small, unless it’s constant | Serious if you keep sending |
| Common cause | Full mailbox, server busy | Address doesn’t exist |
The pattern is simple. A soft bounce says “not right now.” A hard bounce says “not ever.”
What Is a Soft Bounce Email?
Soft bounce emails fail to reach their recipient due to a temporary delivery issue.
The address itself is usually fine. Your message reached the recipient’s mail server, and that server turned it away for now.
Common reasons for soft bounce emails include:
- The recipient’s mailbox is full
- The mail server is temporarily down or offline
- The mail server or mailbox is temporarily overloaded (receiving too many messages at the same time)
- The email is too large for the recipient’s inbox
- The message does not meet the receiving server’s requirements for authentication or sender credentials
Most mail servers retry soft bounces on their own. SendGrid, for example, keeps trying for up to 72 hours before giving up. HubSpot does something similar with what it calls a “pending bounce.”
You usually don’t need to do anything. But repeated soft bounces to the same address are a warning sign, not background noise.
What Is a Hard Bounce Email?
Hard bounce emails are those that have been permanently rejected by the receiving mail server. This means you’ll need to take action before the email can be delivered.
The most common cause is boring: the address doesn’t exist. Usually that’s a typo at signup, or an employee who left the company. Other causes of hard bounces include:
- Failed authentication, so the server can’t verify you sent the message
- A poor sending domain reputation, or your domain or IP address is on a spam blacklist
- Your sending domain is blocked by receiving mail server rules
Because repeatedly resending a hard bounce email can damage your sender reputation, most email service providers will not automatically attempt to resend an email after it hard bounces.
When an email sent to a particular email address bounces for a permanent reason, SendLayer adds this address to a suppression list. This means that even if you attempt to email this address again, the email will not be sent. This will protect your sending domain from being blacklisted while you investigate the issue.
How to Read Your Bounce Message
Every bounce carries a status code. Learning a handful of them turns a vague failure into a specific fix.
Modern bounces use enhanced status codes, written as three numbers like 5.1.1. The first digit is the one that matters most. RFC 3463 defines it clearly:
- 4.X.X is a “persistent transient failure.” The message is valid, but a temporary condition is delaying it.
- 5.X.X is a permanent failure. The spec says it’s “not likely to be resolved by resending the message in the current form.”
So 4 means soft, and 5 means hard. Here are the codes you’ll actually run into.
| Code | What it means | Type |
|---|---|---|
5.1.1 | Bad destination mailbox. The part before the @ is invalid. | Hard |
5.1.2 | Bad destination system. The domain after the @ can’t accept mail. | Hard |
5.2.2 | Mailbox full, over its permanent quota. | Hard |
4.2.2 | Mailbox full, treated as temporary. | Soft |
5.2.3 | Message is longer than the recipient’s size limit. | Hard |
4.4.2 | Bad connection. The transaction timed out mid-send. | Soft |
5.7.1 | Delivery not authorized. Filtering or policy refused you. | Hard |
550 5.7.26 | Gmail rejected an unauthenticated message. | Hard |
421 | Receiving service is unavailable. | Soft |
450 | Recipient’s mailbox is unavailable. | Soft |
452 | Receiving server is overloaded or out of storage. | Soft |
550 | Generic permanent failure, often an unknown user. | Hard |
554 | Transaction failed. Often means you’re blacklisted. | Hard |
For a fuller list, see our guide to SMTP server error codes.
Why Bounce Types Aren’t Always Clear-Cut
The 4-means-soft, 5-means-hard rule is a good default. It isn’t a guarantee.
Mailbox providers don’t apply the codes consistently. Twilio says so about its own bounce handling. In its words, “not all ISPs adhere to that code consistently.”
Two things make this messier in practice.
Soft bounces become hard bounces. Most providers convert an address after enough failures. Mailchimp allows 7 soft bounces for an unengaged address. Contacts who’ve been active before get up to 15. After that, it’s a hard bounce and the address gets cleaned.
The same reason can be classed either way. A full mailbox is the clearest example. It shows up as 4.2.2 on one server and 5.2.2 on another. An abandoned account and a busy one look identical from the outside.
The practical takeaway: trust the code class for your first move, then watch the pattern. A single soft bounce means nothing. The same soft bounce five times means the address is probably dead.
How to Fix a Bounced Email
As there’s not one single reason for an email resulting in a hard bounce or soft bounce, you’ll need to check the SMTP error code and message for more information that can help you to solve the problem.
If the address is invalid (5.1.1, 550): Check for a typo first. Reaching out through another channel to confirm the address is often faster than guessing. If you can’t confirm it, remove it from your list.
If the mailbox is full (4.2.2, 5.2.2): Wait. Your provider will retry. If it keeps failing, treat the address as inactive.
If the server is overloaded (452): You may be sending more emails than your server can handle. If your emails are frequently bouncing with this error, consider upgrading to a dedicated provider like SendLayer.
If your content was flagged: Look for “spam” or “suspicious” in the error text. Review the copy for spam trigger words, and check any links you’ve included.
If authentication failed (5.7.1, 550 5.7.26): Add SPF, DKIM, and DMARC records to your DNS. It’s the bounce we see most often from senders who skipped authentication. It’s also the easiest one to fix. Gmail in particular blocks unauthenticated mail outright.
If you’re blacklisted (554): Stop sending. Check your domain against the major blocklists, then request delisting once you’ve fixed the underlying problem.
What’s a Good Email Bounce Rate?
Your bounce rate is the share of sent messages that bounced.
It’s almost impossible to achieve a bounce rate of 0 if you’re sending to a large list. Aim to keep it under 2%. Postmark’s deliverability benchmarks treat anything above 3% as a problem worth investigating.
What matters more is the mix. A 1%% rate of pure hard bounces beats a 2%% rate of soft bounces. Hard bounces mean bad data, and bad data compounds.
Two other numbers are worth watching alongside it. Google asks senders to keep spam complaints below 0.3%, and recommends under 0.10%. Our guide to monitoring deliverability with Postmaster Tools walks through tracking that figure. Postmark treats open rates above 20% as healthy. Under 10% is a reason to look closer.
How to Check Which Emails Bounced
You can’t fix a bounce you can’t see. There are three places to look.
Your sending service’s logs. This is the most direct option. Good logs show the recipient, the status code, and the raw server response for each failed message. They also let you tell a delivered message from an accepted one.
Your email marketing platform. Most platforms report bounce rate per campaign and split hard from soft. Mailchimp keeps bounce receipts for 30 days, so don’t rely on them as a long-term record.
Google Postmaster Tools. This covers Gmail traffic only, but it’s free. It’s also the closest thing to Gmail’s own view of you. Use it to track your spam complaint rate and domain reputation.
Check after every large send. Bounce problems are much cheaper to fix early.
How to Improve Your Email Bounce Rate
The best way to reduce your email bounce rate is by following email deliverability and list hygiene best practices. This includes:
- Regularly cleaning your email lists to avoid sending email campaigns to invalid email addresses.
- Using a double-optin to confirm email addresses when users subscribe to your mailing list.
- Including a clear unsubscribe link in marketing emails.
- Using segmentation to ensure your marketing emails are sent to a relevant audience, and making sure not to include marketing messages in transactional emails.
- Warming up your email domain before sending marketing emails.
- Monitoring your email deliverability stats and metrics so you are aware of hard and soft bounce emails and can take appropriate action to fix the problems.
Meet the Current Sender Requirements
Authentication used to be optional in practice. It isn’t anymore, and unauthenticated mail now hard bounces at the two largest mailbox providers.
Google and Yahoo, since February 2024. Send 5,000 or more messages a day to Gmail, and you need SPF, DKIM, and DMARC. Marketing mail also needs one-click unsubscribe headers. Keep your Postmaster Tools spam rate below 0.3%.
Microsoft Outlook, since May 5, 2025. The same 5,000-a-day threshold applies to outlook.com, hotmail.com, and live.com. SPF and DKIM both have to pass. DMARC needs to be set to at least p=none, aligned with SPF or DKIM.
There’s a timing trap here worth knowing about. A standard domain warmup schedule crosses 5,000 sends a day somewhere around week three or four. That’s exactly when both rulesets start to apply.
So authentication needs to be in place before your first send, not added once you hit volume.
How SendLayer Helps You Manage Bounces
One of the most effective ways to improve your email deliverability rate and reduce high bounce rates is by using an SMTP email delivery service like SendLayer.
SendLayer’s domain authentication, detailed email logs, and suppression list will help you monitor and manage your outgoing emails so you can instantly spot hard and soft bounce emails and either fix the problem or remove the recipient from your list.
Detailed logs show you which messages bounced and which code came back. You get to act on a specific error instead of a general trend.
Frequently Asked Questions About Bounced Emails
What’s the main difference between a hard bounce and a soft bounce?
A soft bounce is temporary and usually retries by itself. A hard bounce is permanent, and you need to remove or fix the address.
Does a soft bounce hurt my sender reputation?
A single soft bounce doesn’t. Repeated ones to the same address do. Most providers eventually reclassify them as hard bounces.
How many soft bounces before an address becomes a hard bounce?
It depends on the provider. Mailchimp allows 7 for an unengaged contact and up to 15 for a previously active one.
Can I resend an email that hard bounced?
You shouldn’t, unless you’ve corrected the address. Resending to a dead address damages your sender reputation and can get your domain blocked.
Why do my emails bounce when the address is correct?
Authentication is the usual culprit. If SPF, DKIM, or DMARC aren’t set up, Gmail and Outlook may reject your mail outright.
What bounce rate is too high?
Keep it under 2%. Above 3%, treat it as a deliverability problem and audit your list.
That’s it! Now you know what hard bounce and soft bounce emails are.
Next, would you like to learn more about email errors? Check out our guide to SMTP server error codes for more information.
