Understanding Bounce Messages in Email
What Are Bounce Messages?
When you send an email, it travels through mail servers before reaching the recipient. If a server cannot deliver your email, it will send back a bounce message to let you know what went wrong.
Think of it as the server saying:
❌ “We couldn’t deliver this email, and here’s why.”
Types of Bounce Messages
Hard Bounce (Permanent failure)The email will never be delivered.
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Soft Bounce (Temporary failure)The delivery failed, but it may work later.
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Common Examples
Address or Domain Issues
| Error Message | What It Means | What You Can Do |
|---|---|---|
550 5.1.1 User unknown | The email address doesn’t exist. | Check for typos or confirm the recipient’s email. |
550 5.1.2 Host unknown | The domain doesn’t exist. | Ensure the domain (e.g. example.com) is correct. |
550 5.1.0 Address rejected | The server doesn’t recognise the address. | Confirm the recipient’s address is valid. |
Mailbox Problems
| Error Message | What It Means | What You Can Do |
|---|---|---|
421 4.2.1 Mailbox full | The recipient’s inbox is full. | Wait and try again later. |
450 4.2.0 Mailbox unavailable | The mailbox is temporarily disabled or blocked. | Try later or contact the recipient. |
452 4.2.2 Insufficient system storage | The recipient’s mailbox or system ran out of space. | Wait until the issue is resolved. |
Server or Connection Issues
| Error Message | What It Means | What You Can Do |
|---|---|---|
450 4.2.0 Greylisting | The server is temporarily rejecting the email. | Usually clears up; try again later. |
451 4.4.1 Connection timed out | The recipient’s server isn’t responding. | Retry later; persistent errors mean the server may be down. |
421 4.4.2 Connection dropped | The connection was unexpectedly closed. | Retry later. |
421 4.7.0 Temporary server error | A temporary issue on the recipient’s server. | Wait and try again later. |
Message Size & Content Issues
| Error Message | What It Means | What You Can Do |
|---|---|---|
552 5.2.3 Message size exceeds fixed limit | Your email (including attachments) is too large. | Reduce attachment size and resend. |
552 5.3.4 Message size limit exceeded | The recipient’s system has stricter size limits. | Compress attachments or use file-sharing services. |
554 5.6.0 Invalid message content | The message format is not accepted (e.g. corrupted headers). | Check formatting or resend with plain text. |
Security & Policy Blocks
| Error Message | What It Means | What You Can Do |
|---|---|---|
550 5.7.1 Relaying denied | The server won’t pass on the message. | Check outgoing server settings or authentication. |
550 5.7.1 Message rejected as spam | The server flagged your message as spam. | Reword the message or remove suspicious attachments. |
550 5.7.25 SPF validation failed | Sender Policy Framework (SPF) check failed. | [Domain Mailbox] Ask your email admin to review DNS SPF settings. |
550 5.7.26 DKIM signature invalid | DomainKeys Identified Mail (DKIM) check failed. | [Domain Mailbox] Ask your email admin to fix DKIM records. |
550 5.7.27 DMARC policy failure | Domain’s DMARC policy rejected the mail. | [Domain Mailbox] Contact your domain/email admin to review DMARC setup. |
Why This Matters
Bounce messages help you:
- Understand if the problem is with the email address or the recipient’s server.
- See whether the error is temporary or permanent.
- Decide on the next step: retry, correct the address, or reach out another way.
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