SMTP Error 451 4.7.23 – Email Blocked Due to SPF Failure: How to Fix It

What is SMTP Error 451 4.7.23?

SMTP (Simple Mail Transfer Protocol) errors refer to errors encountered in the process of sending mails. SMTP Error 451 4.7.23 is an indication that the receiving mail server has temporarily refused the email because it could not verify that the sending server is authorized to send email from the domain.

This occurs when the SPF record—one of two important DNS authentication methods—isn’t set up correctly or does not include the mail server sending out the email.

SPF failures can even cause real emails to be treated as spam or phishing attempts and thus affect business communications, sales outreach, or customer engagement. Repeated SPF failures, if left unaddressed, can result in an impaired email reputation, lowering the likelihood that your emails will land in inboxes down the line.

Why SPF authentication matters for email security and deliverability

SPF is an email authentication protocol used to prevent and combat email spoofing, a technique that attackers and spammers regularly use to pretend to be mail servers belonging to trusted organizations. When an email is sent, the mail server of the recipient queries whether the sending IP sits behind the sender domain’s SPF record If the check fails, the email may be:

• Rejected (SMTP Error 451 4.7.23)
• Marked as spam
• Quarantined or delayed

SPF authentication plays a crucial role in:

• Enhancing email security by protecting against domain spoofing and phishing attacks
• Improving email deliverability by identifying legitimate senders
• Maintaining good sender reputation with email service providers (ESPs)

However, SPF failures can happen due to misconfigured DNS records, too many SPF lookups, email forwarding, or unauthorized third-party senders. Knowing the causes and finding the correct solution is the key to fixing email deliverability.

Common causes of SPF failures leading to Error 451 4.7.23

If your email server fails SPF authentication, the recipient’s mail server may return SMTP Error 451 4.7.23, temporarily rejecting the email due to SPF policy violations. Below are the most common causes of SPF failures and how they lead to this error.

Missing or misconfigured SPF record

If a domain does not have an SPF record, recipient mail servers have no way of verifying whether the sending server is authorized. A misconfigured SPF record can also lead to failures if:

• The SPF record contains syntax errors
• The record does not include all necessary email servers
• Multiple SPF records exist (SPF only supports one record per domain)

Example of a correct SPF record format: A correctly formatted SPF record should follow a structured syntax. Below is an example of a standard SPF record:

v=spf1 ip4:192.168.1.1 include:_spf.google.com -all  

Exceeding SPF DNS lookup limit

SPF records cannot exceed 10 DNS lookups. This is a strict limit set to prevent SPF validation from causing excessive DNS queries, which could slow down email processing. Every include, a, mx, and ptr mechanism in an SPF record counts as a lookup. If you exceed 10 lookups, the recipient’s mail server will automatically fail SPF validation, even if the record is correct.

Email forwarding and SPF failures

SPF relies on checking the original sending IP address. However, when an email is forwarded, the return path changes, and the forwarder’s server becomes the “new sender.” Since the forwarder’s IP is not included in the original SPF record, SPF fails, leading to SMTP Error 451 4.7.23.

When an email is forwarded, SPF validation often fails because:

• The recipient’s mail server checks the forwarder’s IP instead of the original sender
• The forwarder is not authorized in the SPF record
• If DMARC (Domain-based Message Authentication, Reporting & Conformance) is strict, SPF failures lead to email rejection

4. Unauthorized third-party email senders

Most businesses are using third-party services to send emails like email marketing tools, CRM systems, automation tools, etc. It means if these services are not on your SPF record, all the emails sent by them will fail SPF checks and may actually get blocked.

5. Conflict with other authentication protocols (DKIM & DMARC)

SPF is just one part of email authentication. It works best when combined with:

• DKIM (DomainKeys Identified Mail): Provides emails with a cryptographic signature that authenticates the emails
• DMARC (Domain-based Message Authentication, Reporting & Conformance): Specifies how SPF and DKIM failures should be processed. Now, if DMARC is p=reject and still failing, the email gets dropped.

🔖 Related Reading: SPF, DKIM, and DMARC: Boosting Email Security and Deliverability

How to fix SMTP Error 451 4.7.23 and SPF failures

There is nothing beneficial or advantageous about SMTP errors. So if you get this error code, it’s not the end of the world—and there are things you can do to work around it. 

Step 1: Verify your SPF record

Before editing anything, you must first verify that you have an existing SPF record in place and also to see if it is configured properly. Use the nslookup command in the terminal to check your SPF record:

Windows/macOS/Linux:

nslookup -type=TXT yourdomain.com

This will return the SPF record (if one exists) associated with your domain.

Example output:

yourdomain.com text = “v=spf1 ip4:203.0.113.5 include:_spf.google.com -all”

If no SPF record is found or it is incorrect, you need to create or update it in your DNS settings. Alternatively, you can use online SPF validation tools such as Warmy’s Free SPF Record Generator.

Step 2: Reduce DNS lookups in SPF

Since SPF allows only 10 DNS lookups, exceeding this limit causes SPF failures. If your SPF record contains too many include mechanisms, you need to reduce the number of lookups through SPF flattening. 

Step 3: Address forwarding-related SPF failures

SPF fails when emails are forwarded because the recipient’s server sees the forwarder’s IP instead of the original sender’s IP. To fix this issue:

• Implement DKIM (DomainKeys Identified Mail). DKIM signs outgoing emails so they remain valid even if forwarded. Ensure your email provider supports DKIM signing.
• Use DMARC to allow SPF failures for forwarded emails. DMARC allows you to specify “relaxed” policies that do not reject forwarded emails.
• Enable ARC (Authenticated Received Chain). Some email providers support ARC headers, which help preserve SPF authentication across email forwards.

If SPF failures persist after forwarding, whitelisting trusted forwarders on the recipient side may help.

Step 4: Authorize third-party email services

If you use email marketing tools, CRM platforms, or transactional email services, you must explicitly authorize them in your SPF record. To identify third-party services sending emails on your behalf:

1. Check email headers for unknown mail servers.
2. Review your email provider’s documentation for SPF recommendations.
3. Use DMARC reports to detect unauthorized senders.

Then, you can find the correct SPF include value for the service and add it to your SPF record.

Step 5: Align SPF with DKIM and DMARC for stronger authentication

SPF alone is not enough to fully protect email deliverability. It works best when combined with DKIM and DMARC.

Why DMARC helps enforce email authentication

DMARC ensures emails failing SPF or DKIM are handled properly. It also provides reporting to help detect unauthorized senders.

How to set up a DMARC policy that prevents false positives

1. Start with a relaxed DMARC policy. This monitors failures without blocking emails.
   
   
2. Gradually enforce DMARC:
   
   v=DMARC1; p=quarantine; pct=50; rua=mailto:[email protected];
   • p=quarantine moves 50% of failed emails to spam.
   • rua sends reports of failed emails to your inbox.
3. Fully enforce DMARC once everything is correct. This blocks all unauthorized emails.

Best practices to prevent future SPF errors

Regularly monitor SPF records and DNS changes

SPF records are stored in the Domain Name System (DNS), and any changes to your email service providers, mail server IPs, or domain configuration can affect SPF validation. 

Why SPF records should be monitored regularly:

• Many email services (Google Workspace, Microsoft 365, Mailchimp, SendGrid, etc.) change their SPF policies or IP addresses periodically. You must stay updated and adjust your SPF record accordingly.
• If your organization starts using a new CRM, email marketing platform, or transactional email service, you must update SPF records to reflect the new senders.
• If your domain provider updates DNS settings or a DNS admin accidentally modifies SPF records, email authentication failures can occur.

Now, here’s how you can monitor your SPF records effectively:

• Set up automated DNS monitoring using tools like DNS Spy or MXToolbox.
• Schedule regular SPF record audits (monthly or quarterly).
• Check SPF validation status using:
  • nslookup -type=TXT yourdomain.com
  • MXToolbox SPF Record Lookup
• Review email headers of sent emails to verify SPF pass/fail status.

🚀 Pro tip: Use SPF flattening to prevent SPF lookup failures and simplify management.

Use dedicated subdomains for third-party email senders

Here’s a list of some common services that most of us use

• Marketing platforms (Mailchimp, HubSpot, ActiveCampaign)
• Transactional email providers (SendGrid, Postmark, Amazon SES)
• CRM and customer support tools (Salesforce, Zendesk, Intercom)

If all of the above services send emails from the same domain (yourdomain.com), it complicates SPF management, increases the likelihood and risk of SPF failures, or creates deliverability conflicts with other authentication protocols such as DMARC.

So instead of using yourdomain.com for all email traffic, create subdomains for different purposes.

Here is an example of a subdomain Strategy:

• Marketing emails: mail.yourdomain.com (SPF for Mailchimp, HubSpot)
• Transactional emails: notify.yourdomain.com (SPF for SendGrid, Amazon SES)
• Support emails: support.yourdomain.com (SPF for Zendesk, Intercom)

Having separate SPF, DKIM and DMARC policies for each subdomain minimizes conflicts with authentication and improves deliverability.

🚀 Pro tip: Separate subdomains also isolate email reputation such that if one email stream (e.g.marketing) runs into deliverability issues, it won’t affect critical business emails (e.g. customer support).

Implement DMARC reports to track authentication failures

Even with SPF and DKIM properly configured, issues can still arise if unauthorized servers attempt to send emails on your behalf. This is where DMARC (Domain-based Message Authentication, Reporting & Conformance) helps by:

• Enforcing email authentication: Prevents spoofed emails from being delivered.
• Providing real-time email security insights: Monitors who are sending emails using your domain.
• Helping diagnose SPF/DKIM failures: DMARC reports show which emails failed authentication and why.

DMARC reports show which email senders passed/failed SPF and DKIM. You can view reports using:

• Google Postmaster Tools (https://postmaster.google.com/)
• DMARC Report Analyzers (https://dmarcian.com/)
• Easy DMARC (https://easydmarc.com/)

If reports show SPF failures from unexpected IPs, you should investigate whether unauthorized senders are trying to impersonate your domain, update your SPF record to include legitimate senders, and gradually increase DMARC enforcement (p=quarantine → p=reject).

How Warmy helps prevent SMTP Error 451 4.7.23

While SPF authentication is a critical component of email security and deliverability, it’s not enough on its own to ensure emails consistently reach the inbox. Warmy goes beyond SPF by offering a comprehensive suite of tools designed to optimize email deliverability, protect sender reputation, and improve engagement rates.

Warmy’s SPF record generator

One of the biggest challenges with SPF is creating and maintaining a correct SPF record. Many senders struggle with formatting errors in SPF records, exceeding SPF’s 10 DNS lookup limit, causing failures, or forgetting to include all email-sending services.

Warmy’s Free SPF Record Generator simplifies this process by:

• Automatically generating the correct SPF record based on your email service providers.
• Optimizing SPF structure to prevent lookup failures.
• Ensuring compliance with best practices to avoid SPF misconfigurations.