SMTP error 451: What it means and how to fix it
SMTP error 451 overview:
- SMTP error 451 is a temporary receiving-server issue. Your message was deferred, not permanently rejected.
- Retries usually solve it. Your email server will queue the message and retry automatically.
- The error text hints at the cause. Common triggers include greylisting or server overload.
- If 451 errors persist, strengthen authentication to improve trust with receiving servers.
SMTP error 451 indicates a local error on the receiving server. The server encountered an issue processing your message, but may accept it later. This is typically a temporary condition that resolves itself.
Want fewer avoidable delivery issues?
Use an enterprise-grade DMARC platform to keep authentication aligned and reduce reputation-related bounces across your domains.
What SMTP error 451 means
A 451 error means the receiving server hit a temporary issue while processing your email, so it deferred delivery. Your email server will queue the message and retry automatically.
You will often see 451 alongside text that hints at the cause (for example, greylisting, server overload, or reputation-based deferrals).
Common causes of SMTP error 451
- Server overload: The receiving server is processing too many requests
- Greylisting: The server is deferring first-time senders
- IP reputation: The recipient is temporarily rejecting your email for reputation reasons
Provider-specific 451 errors
Google (Gmail, Google Workspace)
| Error message | Meaning |
|---|---|
| 451 4.3.0 – Mail server temporarily rejected message | Server overload or temporary backend issue |
Yahoo (Yahoo, AOL, Verizon)
| Error message | Meaning |
|---|---|
| 451 example.yahoo.com – Resources temporarily unavailable | Potentially suspicious activity detected |
| 451 4.3.2 – Internal server error | Temporary issue on the receiving server |
Microsoft (Outlook.com)
| Error message | Meaning |
|---|---|
| 451 4.3.2 – Temporary server error | Server load, maintenance, or a network problem |
| 451 4.4.0 – Message failed to be replicated | Server couldn’t complete the request |
| 451 4.7.0 – Temporary server error | Temporary server-side issue |
| 451 4.7.650 – The mail server [x.xx.xx.xx] has been temporarily rate limited due to IP reputation | Low IP reputation triggering rejections |
| 451 4.7.651 – The mail server [x.xx.xx.xx] has been temporarily rate limited due to IP reputation | Reputation-based deferral |
Exchange Online (Microsoft 365)
| Error message | Meaning |
|---|---|
| 451 4.7.0 – Temporary server error | Server-side processing issue |
| 451 4.7.500 – Server busy | Server under heavy load |
| 451 4.4.62 – Mail sent to wrong Office 365 region | Configuration, certificate, or routing issue |
iCloud (icloud.com, me.com, mac.com)
| Error message | Meaning |
|---|---|
| 451 4.7.1 – Service unavailable – try again later | Server can’t accept messages right now |
Yandex
| Error message | Meaning |
|---|---|
| 451 4.7.1 – Sorry, the service is currently unavailable | Maintenance window or high load |
Rackspace
| Error message | Meaning |
|---|---|
| 451 4.7.1 – Received too many messages from a new or untrusted IP | Message has been greylisted |
How to fix SMTP error 451
Step 1: Let automatic retries run
Most 451 errors resolve through automatic retries:
- Your email server queues deferred messages
- Retries typically occur after 30 minutes
- Most servers retry for 48 to 72 hours
If messages are delivered on retry, no action is needed.
Step 2: Check for greylisting
Greylisting temporarily rejects first-time senders to deter spam.
Signs of greylisting:
- SMTP 451 errors often appear on the first contact with a domain
- Messages are delivered successfully on retry
- The error message mentions a new or untrusted IP
Greylisting is normal and doesn’t require any fix.
Step 3: Review IP reputation
Reputation-based 451 errors require attention:
Check your status:
- Microsoft SNDS for Outlook reputation
- Google Postmaster Tools for Gmail reputation
- Blocklist lookup tools
Improve your reputation:
- Authenticate all emails
- Maintain clean mailing lists
- Keep complaint rates below 0.3%
- Monitor bounce rates
Step 4: Contact the recipient (if persistent)
For ongoing 451 errors when sending to a specific recipient domain:
- Contact the recipient through an alternative channel
- Ask their email admin to allowlist your sending domain or IP
How authentication reduces 451 errors
Proper email authentication improves your sender reputation, reducing deferrals.
SPF
SPF (Sender Policy Framework) specifies which servers can send email on behalf of your domain.
Example SPF record:
| Host | Type | Value |
|---|---|---|
@ | TXT | v=spf1 ip4:192.168.0.1 include:mail.example.com -all |
DKIM
DKIM (DomainKeys Identified Mail) adds a cryptographic signature to your messages, so receivers can verify message authenticity.
Example DKIM record:
| Host | Type | Value |
|---|---|---|
selector._domainkey.yourdomain.com | TXT | v=DKIM1; k=rsa; p=[public key] |
DMARC
DMARC (Domain-based Message Authentication, Reporting, and Conformance) tells receivers how to handle authentication failures and provides reporting.
Example DMARC record:
| Host | Type | Value |
|---|---|---|
_dmarc.yourdomain.com | TXT | v=DMARC1; p=reject; rua=mailto:dmarc-reports@yourdomain.com; fo=1; |
Reduce 451 errors with Sendmarc
Missing or misaligned authentication can make email look suspicious and lead to temporary failures. Sendmarc helps you:
- Implement complete authentication to build server trust
- Monitor domain reputation across providers
- Identify unauthorized senders using your domain
- Use DMARC reporting to spot authentication failures
- Improve delivery by resolving issues proactively
Use Sendmarc’s enterprise-grade platform to keep authentication aligned, improve trust with receiving servers, and reduce avoidable 451 deferrals.
SMTP error 451 FAQs
How long will the receiving server keep rejecting with 451?
A receiving server will usually stop rejecting with a 451 error once the temporary condition clears. If the 451 is caused by greylisting, it often clears within five to 30 minutes. If the 451 is caused by a broader temporary issue (like server overload), it typically resolves within a few hours.
Should I remove addresses that get 451 errors?
No, you shouldn’t remove addresses just because they return a 451 error. A 451 error is a temporary failure (a 4xx response), so your server will usually retry delivery automatically, and messages will often be accepted once the issue clears.
Do I need to resend an email after a 451 error?
No. A 451 error is a temporary deferral, so your email server will usually queue the message and retry delivery automatically.