Every SMTP 5xx Rejection Code Explained with Fixes

Understanding 5xx Codes

SMTP 5xx codes indicate permanent failure. The receiving server will not accept this message — ever — unless something changes on your end.

Structure: 5XY.Z.Z.Z

• 5 = Permanent failure
• XY = Category
• Z.Z.Z = Enhanced status code (detailed reason)

550 Codes: Mailbox Unavailable / Policy Rejection

550 5.1.1 — User Unknown

Meaning: The email address doesn't exist.

Common messages:

• "The email account that you tried to reach does not exist"
• "User unknown"
• "No such user"

Fixes:

1. Verify the email address is correct (typos?)
2. Remove from list — this is a hard bounce
3. Contact recipient through other channels to confirm address
4. Implement email validation before adding addresses to list

550 5.1.0 — Address Rejected

Meaning: The address format is invalid or rejected.

Fixes:

1. Check address format (missing @, invalid characters)
2. Verify domain exists
3. May indicate domain-level block

550 5.7.1 — Policy Rejection

Meaning: Message rejected for policy reasons.

Common causes:

• IP or domain on blacklist
• Content flagged as spam
• Failed authentication
• Sending volume too high
• Poor sender reputation

Fixes:

1. Check IP against Spamhaus, MXToolbox
2. Verify SPF, DKIM, DMARC
3. Review content for spam triggers
4. Check Google Postmaster Tools reputation
5. Reduce sending volume if new IP/domain

Practitioner note: 550 5.7.1 is the most common rejection I see, and it's frustratingly vague. Start with blacklist checks and authentication — those are the quick wins. If both look fine, it's usually reputation or content.

550 5.7.26 — DMARC Failure

Meaning: Email failed DMARC authentication and sender's policy says to reject.

The math: Neither SPF nor DKIM passed with alignment, and your DMARC policy (or theirs) is at p=reject.

Fixes:

1. Check DMARC authentication
2. Verify SPF includes the sending IP
3. Confirm DKIM is configured and aligned
4. Review DMARC reports for failure details

550 5.7.0 — Authentication Required

Meaning: Server requires authentication that wasn't provided.

Fixes:

1. Enable SMTP authentication
2. Verify credentials are correct
3. Check if server requires TLS

550 5.7.25 — PTR Record Failure

Meaning: Sending IP lacks valid reverse DNS (PTR record).

Fixes:

1. Set up PTR record for sending IP
2. PTR should resolve to your mail server hostname
3. Forward lookup should resolve back to the same IP

550 5.4.1 — Recipient Address Rejected

Meaning: Server won't accept mail for that address.

Causes:

• Mailbox disabled
• Account suspended
• Domain not hosted on that server

Fixes:

1. Contact recipient to verify account status
2. Try alternative address
3. Remove from list if persistent

551 Codes: User Not Local

551 5.1.6 — User Has Moved

Meaning: The mailbox has moved to a different server.

The message usually includes the new address.

Fixes:

1. Note the forwarding address if provided
2. Update your records
3. Send to new address

552 Codes: Storage Issues

552 5.2.2 — Mailbox Full

Meaning: Recipient's mailbox is over quota.

Fixes:

1. This is technically a temporary issue
2. Most ESPs will retry a few times
3. If persistent, the mailbox may be abandoned
4. Consider removing after multiple failures over time

552 5.3.4 — Message Too Large

Meaning: Your message exceeds the server's size limit.

Fixes:

1. Reduce attachment sizes
2. Use file sharing links instead of attachments
3. Compress attachments
4. Check receiving server's published limits

553 Codes: Address Issues

553 5.1.3 — Invalid Address Syntax

Meaning: The email address format is malformed.

Fixes:

1. Check for special characters
2. Verify address format
3. Remove or correct invalid addresses

553 5.7.1 — Sender Not Authorized

Meaning: You're not allowed to send as this address.

Causes:

• SPF failure
• Relay not permitted
• Authentication issues

Fixes:

1. Verify you're using authorized sending address
2. Check SPF record includes your IP
3. Authenticate properly with the server

554 Codes: Transaction Failed

554 5.7.1 — Blocked

Meaning: Generic "your email is blocked."

Usually accompanied by:

• Blacklist references
• Reputation warnings
• Policy links

Fixes:

1. Check all major blacklists
2. Review authentication
3. Check sender reputation
4. Follow provider's unblock process if listed

554 5.7.5 — Cryptographic Failure

Meaning: TLS or encryption issue.

Fixes:

1. Update TLS to 1.2 or higher
2. Check certificate validity
3. Verify cipher suite compatibility

554 5.7.9 — Permission Denied

Meaning: Server explicitly denied your connection.

Fixes:

1. Check if your IP is banned
2. Review any previous abuse reports
3. Contact postmaster

Provider-Specific 550 Errors

Gmail

Code	Meaning
550 5.7.26	DMARC failure
550 5.7.1	Policy block (reputation/blacklist)
550 5.1.1	User doesn't exist
550 5.2.1	Account disabled

Outlook/Microsoft 365

Code	Meaning
550 5.7.606	Blocked by admin
550 5.7.708	Not allowed due to policy
550 5.4.1	Recipient rejected
550 5.7.1	Sender blocked

Yahoo

Code	Meaning
553 5.7.1	Spam policy rejection
553 5.7.2	Sending limit exceeded
554 5.7.9	Permission denied

Decision Tree

5xx Error Received
├── 550 5.1.x (Address) → Verify address, remove if invalid
├── 550 5.7.x (Policy)
│   ├── Check blacklists first
│   ├── Verify authentication (SPF, DKIM, DMARC)
│   ├── Review sender reputation
│   └── Check content for spam triggers
├── 551 (User moved) → Update to new address
├── 552 (Storage) → Retry later or remove stale addresses
├── 553 (Format) → Fix address syntax
└── 554 (Blocked) → Deep investigation needed

Logging and Tracking

Keep records of 5xx bounces:

1. Log the exact code and message — provider-specific details matter
2. Track patterns — same error from multiple IPs = your problem
3. Remove hard bounces immediately — continued attempts hurt reputation
4. Investigate soft patterns — 552 once is fine, 552 repeatedly suggests abandoned mailbox

Practitioner note: I've seen senders blast the same invalid addresses for months because their bounce handling was broken. Every failed send makes your reputation worse. Fix your bounce processing before worrying about anything else.

If you're seeing widespread 5xx rejections and can't identify the cause, schedule a consultation — I'll analyze your bounce logs and pinpoint the issue.

Sources

• RFC 5321: Simple Mail Transfer Protocol
• RFC 3463: Enhanced Mail System Status Codes
• Google: Email error messages
• Microsoft: Non-delivery reports

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v1.0 · March 2026