The 451 “Message Temporarily Deferred” error (with DKIM-related messaging) means receiving servers are throttling your email because DKIM authentication is missing.
Fix it by generating a DKIM key through your email provider, publishing the TXT record in DNS, and configuring SPF and DMARC for complete authentication.
Major providers (Yahoo, Microsoft, and increasingly Gmail) defer or reject unauthenticated mail. The 451 code indicates a temporary deferral — recipients aren’t blocking you permanently, but they’re signaling that authentication issues must be resolved.
Without DKIM, you’re essentially sending mail without proper identification (and providers treat unknown senders cautiously).
Quick skim — 451 error overview
The 451 DKIM deferral indicates authentication gaps that major providers increasingly penalize.
| Attribute | Details |
| Error code | 451 (DKIM variant) |
| Category | Authentication failure |
| Meaning | Email deferred due to missing DKIM |
| Severity | Temporary (requires authentication setup) |
| Common causes | DKIM not configured, DNS record missing, provider switch |
| Fix approach | Generate key → publish DNS → verify → wait for propagation |
What does DKIM deferral mean?
DKIM (DomainKeys Identified Mail) cryptographically signs emails, proving they originated from authorized senders and weren’t modified in transit. Without DKIM, receiving servers can’t verify message authenticity — so they defer delivery while evaluating other signals.
Why do providers require DKIM
Authentication prevents spoofing and builds trust:
| With DKIM | Without DKIM |
| Message integrity verified | No tampering protection |
| Sender identity confirmed | Unknown origin |
| Reputation applies accurately | Reputation unclear |
| Often bypasses scrutiny | Subject to throttling |
Yahoo and Microsoft specifics
Yahoo explicitly defers mail lacking DKIM:
- Bulk senders must have DKIM
- Missing authentication triggers 451 responses
- Persistent issues escalate to permanent rejection
Microsoft increasingly follows similar patterns, particularly for high-volume senders.
Why does a 451 error occur?
DKIM deferrals stem from configuration gaps at the DNS or mail server level.
DKIM never configured
Most common — authentication simply wasn’t set up:
- Domain launched without DKIM
- Email service used without completing setup
- Assumed authentication was automatic (it rarely is)
DNS record missing
DKIM key might be generated but not published:
- Record never added to DNS
- Typo in the selector or the domain
- Record deleted accidentally
Provider switch
Changing email providers invalidates old DKIM:
- Each provider uses different keys
- Old DNS records become invalid
- New provider’s records weren’t added
Rate limiting
Volume spikes without authentication trigger stricter deferrals:
- Sudden increase in sending
- New domain without warmup
- Behavior resembled spam patterns
How do you fix DKIM deferral?
Complete DKIM setup resolves the deferral. Additionally, configure SPF and DMARC for comprehensive authentication.
Generate DKIM key
Access your email provider’s authentication settings:
Google Workspace
- Admin Console → Apps → Google Workspace → Gmail
- Navigate to “Authenticate email.”
- Generate DKIM key (select bit length — 2048 recommended)
- Copy provided TXT record
Microsoft 365
- Defender portal → Email authentication → DKIM
- Select your domain
- Enable DKIM signing
- Copy DNS records provided
Other providers
- Locate authentication or deliverability settings
- Generate DKIM selector and key pair
- Copy the TXT record for DNS publication
Publish DNS record
Add the DKIM public key to your domain’s DNS:
- Log in to your DNS provider (Cloudflare, GoDaddy, etc.)
- Add TXT record with format: selector._domainkey.yourdomain.com
- Paste the full key value from your email provider
- Save changes
The selector (like google, selector1, s1) comes from your email provider — use exactly what they specify.
Configure SPF
Complement DKIM with SPF authorization:
- Add TXT record for your domain
- Include all authorized sending IPs
- Stay under 10 DNS lookups
- End with -all or ~all
Use the SPF generator for proper formatting.
Configure DMARC
Add policy layer for complete authentication:
- Create TXT record at _dmarc.yourdomain.com
- Start with p=none while monitoring
- Enable reporting with rua= tag
Use the DMARC generator for correct syntax.
Wait for propagation
DNS changes require time to spread:
- Typically 15 minutes to 4 hours
- Occasionally up to 48 hours
- Test before assuming failure
Verify configuration
Confirm everything works:
- Use DKIM lookup to verify the record published
- Send a test email and check the Authentication-Results header
- Run email deliverability test across providers
How do you prevent future DKIM issues?
Maintaining authentication prevents recurring deferrals.
Document configuration
Track authentication setup:
- Record selector names and key generation dates
- Note which DNS provider hosts records
- Document provider-specific settings
Rotate keys periodically
DKIM keys should rotate annually:
- Generate a new key with a new selector
- Publish the new record before switching
- Update signing configuration
- Keep the old selector active during the transition
Verify after changes
Test authentication whenever infrastructure changes:
- Provider migrations
- DNS provider switches
- Domain registrar transfers
Still stuck after trying the fix?
Some email errors are easy to clear. Others point to deeper deliverability issues involving authentication, sender reputation, blacklisting, routing, or mailbox provider policy. If you would rather have an expert review it, speak with an email delieverability consultant for free and we can help diagnose the issue and fix it on your behalf.
We look beyond the error message itself to find what is actually breaking delivery, trust, or inbox placement.
From SPF, DKIM, and DMARC to blacklist cleanup, DNS alignment, and sending setup, we can guide or implement the fix.
We assess whether the error is part of a bigger pattern hurting opens, replies, and overall campaign performance.
Talk to a real deliverability expert, get honest guidance, and see the next best step without pressure or upsells.
When should you book a consultation? If the error keeps coming back, affects multiple mailboxes or domains, started after an ESP or DNS change, or is tied to spam placement, low inboxing, high bounce rates, or authentication failures, it is usually faster to get an expert involved early.
Frequently asked questions
Here are some commonly asked questions about this error:
Once DNS propagates (typically 15 minutes to 4 hours), properly signed emails should be delivered immediately. Existing queued messages may still fail — they were sent before DKIM was configured. New messages should succeed.
DKIM significantly improves deliverability, but combine with SPF and DMARC for the best results. Some providers require all three for bulk senders. Authentication alone doesn’t guarantee delivery — reputation, content, and list quality also matter.
Your mail server retries automatically (for 451 errors). Messages in the queue will attempt delivery again. If using an ESP, check their retry policy — some require manual resend for failed campaigns.
