How to fix Gmail errors for SPF authentication, unauthenticated senders, and PTR records?
Michael Ko
Co-founder & CEO, Suped
Published 20 Jun 2025
Updated 29 Aug 2025
7 min read
Email deliverability can be a constant challenge, and the recent changes by major inbox providers like Gmail have only made it more complex. I've noticed an uptick in specific error messages that indicate a fundamental problem with how emails are authenticated: SPF authentication failures, messages blocked due to unauthenticated senders, and issues with PTR records (reverse DNS).
These errors, such as 421-4.7.27 (SPF rate limiting), 550-5.7.26 (unauthenticated sender), and 421-4.7.23 (PTR record mismatch), often mean your emails are either being temporarily blocked or outright rejected. While sometimes these can be random network glitches, they more often point to misconfigurations in your DNS records.
For high-volume senders, even a tiny percentage of these failures can lead to significant deliverability problems and impact your sender reputation. Understanding and correctly implementing email authentication protocols is no longer optional, it's a fundamental requirement for reaching your audience's inboxes. Let's delve into how to diagnose and rectify these critical Gmail errors.
Gmail, like other major mailbox providers, relies heavily on email authentication to combat spam and phishing. SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance) are the cornerstones of this authentication system. If these aren't set up correctly, your emails are at high risk of being rejected or sent to spam.
An SPF record specifies which mail servers are authorized to send email on behalf of your domain. Without a proper SPF record, receiving servers can't verify that your emails are legitimate, leading to unauthenticated sender errors. DKIM provides a digital signature, ensuring the email hasn't been tampered with in transit. DMARC builds on SPF and DKIM, telling receiving servers what to do if an email fails authentication, and provides valuable DMARC reports that reveal authentication issues. For guidance on setting these up, check out how to set up SPF, DKIM, and DMARC.
Correct configuration of these records is vital. A common mistake is having multiple SPF records or exceeding the 10-DNS-lookup limit. These issues prevent email servers from properly authenticating your messages, leading to the blocks you're seeing.
Fixing SPF authentication and unauthenticated sender errors
The 550-5.7.26 error is a direct indication that Gmail couldn't verify your sender, meaning your SPF or DKIM setup, or both, are likely incorrect. The 421-4.7.27 error, while a temporary deferral, also points to SPF issues potentially leading to rate limiting because of failed authentication. To fix these, you need to thoroughly review your SPF record.
Steps to fix SPF authentication errors
Verify your SPF record: Ensure you have exactly one SPF TXT record for your domain. Multiple records will cause authentication to fail. Use an online SPF checker to validate its syntax and included mechanisms.
Include all authorized senders: Every service that sends email on behalf of your domain (e.g., Mailchimp, SendGrid, Salesforce) must be included in your SPF record. For example, don't add MailChimp to your SPF record incorrectly.
Mind the 10-DNS-lookup limit: Your SPF record can only have up to 10 DNS lookups. Exceeding this limit will cause SPF to fail, leading to authentication errors. Regularly review your record to ensure compliance.
Set a proper SPF policy: Use -all for a strict policy that rejects unauthorized emails. For monitoring, you might start with ~all, but eventually move to -all once your senders are fully authorized. You can troubleshoot SPF failures with Google Postmaster Tools.
After adjusting your SPF record, it’s crucial to implement DMARC. DMARC gives you visibility into your email authentication status through aggregate and forensic reports, showing you exactly which emails are passing or failing SPF and DKIM. This is where a tool like Suped becomes invaluable, offering the most generous free plan for DMARC reporting and monitoring to help you pinpoint and fix these authentication issues.
Addressing PTR record (Reverse DNS) issues
The error message 421-4.7.23 specifically points to a problem with your PTR (Pointer) record, also known as reverse DNS. A PTR record translates an IP address back into a domain name. Gmail and other major mail providers use PTR records to verify that the sending IP address matches the domain name it claims to be from, adding another layer of trust to your emails.
If your sending IP lacks a valid PTR record, or if the forward DNS (A record) doesn't match the reverse DNS, Gmail will often temporarily block or even reject your emails. This is a common requirement for legitimate mail servers to combat spam, as spammers often use IP addresses without proper reverse DNS to hide their identity. You can resolve Gmail PTR record errors by working with your hosting provider.
PTR record issues
Missing PTR record: The sending IP address does not have a corresponding PTR record defined in DNS.
Mismatch between forward and reverse DNS: The hostname obtained from the PTR record doesn't match the hostname in the forward (A) record for the sending IP. For more details, consult Google's Email sender guidelines.
Temporary DNS failures: Transient network or DNS resolution problems can lead to intermittent PTR record lookup failures.
Solutions
Contact your hosting or ISP: PTR records are managed by the entity that controls the IP address block. You'll need to request that they set up or correct your reverse DNS record to match your sending domain's hostname.
Ensure consistent DNS entries: Make sure the forward (A) record for your mail server's hostname points to the correct IP, and the PTR record for that IP points back to the same hostname.
Use a dedicated sending IP: If you share an IP address, coordinating PTR records can be difficult. A dedicated IP gives you full control over reverse DNS settings.
Once your PTR record is configured, it can take some time for DNS changes to propagate globally. It's a good practice to use a DNS lookup tool to confirm that your PTR record is correctly resolving.
Proactive monitoring and ongoing maintenance
Dealing with these Gmail errors isn't a one-time fix, it requires ongoing vigilance and proactive monitoring. DNS issues can arise unexpectedly, and authentication standards continue to evolve. This is why having a robust DMARC monitoring system in place is not just recommended, but essential. Suped offers DMARC monitoring that provides comprehensive insights into your email authentication, helping you quickly identify and resolve issues before they impact your deliverability.
Regularly reviewing your DMARC reports helps you understand how Gmail and other receivers are handling your emails, allowing you to catch SPF, DKIM, or alignment failures early. Additionally, keeping an eye on your domain reputation through tools like Google Postmaster Tools and monitoring for any blocklist (or blacklist) appearances can prevent unexpected delivery interruptions.
Implementing a DMARC policy of p=quarantine or p=reject is the ultimate goal, protecting your domain from spoofing and significantly improving your email trust. Suped helps you achieve this by simplifying the path to DMARC enforcement and providing clear, actionable insights from your reports. Consider using our free DMARC record generator tool to get started.
Views from the trenches
Best practices
Always maintain a single, accurate SPF TXT record per domain to avoid authentication conflicts.
Regularly review your DMARC aggregate reports to identify authentication failures proactively.
Ensure that all sending IPs have correctly configured PTR records that match forward DNS.
Common pitfalls
Having multiple SPF records, which causes SPF validation to fail intermittently or consistently.
Exceeding the 10-DNS-lookup limit within your SPF record, leading to authentication errors.
Neglecting PTR record configuration, resulting in temporary blocks from major mailbox providers.
Expert tips
Use a DMARC reporting tool like Suped to monitor authentication results and pinpoint issues.
When encountering unexpected errors, always double-check DNS settings first, as it's a frequent culprit.
Understand that some minor, intermittent failures can be due to global DNS or network 'internetting' that is beyond your control.
Expert view
Expert from Email Geeks says if SPF passes and rDNS is correctly configured, you should investigate your DNS infrastructure for any issues that might be preventing proper resolution.
2024-08-20 - Email Geeks
Expert view
Expert from Email Geeks states that there aren't widespread global issues with Gmail; it's best to assume the error messages are accurate and use them as a guide.
2024-08-20 - Email Geeks
Your path to better Gmail deliverability
Navigating Gmail's stringent email authentication requirements can seem daunting, but addressing SPF, unauthenticated sender, and PTR record errors is crucial for reliable email delivery. By meticulously configuring your SPF records, implementing DKIM, and ensuring your PTR records are correctly set up, you significantly enhance your domain's credibility and reduce the chances of your emails being blocked or flagged as spam.
Remember, email deliverability is an ongoing process. Continuous monitoring of your authentication performance through DMARC reports is key to staying ahead of potential issues. Tools like Suped, with its generous free DMARC reporting plan, provide the visibility you need to diagnose and fix these problems efficiently, ensuring your messages consistently reach their intended recipients.
By taking these steps, you not only resolve current Gmail errors but also build a resilient email sending infrastructure that supports long-term deliverability and sender reputation.
