What causes SPF verification failures for emails forwarded to Gmail?
Michael Ko
Co-founder & CEO, Suped
Published 23 May 2025
Updated 18 Aug 2025
8 min read
Email forwarding can be incredibly convenient, allowing you to centralize mail from multiple addresses into a single inbox, like Gmail. However, this seemingly simple act often leads to a complex problem: SPF (Sender Policy Framework) verification failures. These failures can result in emails being marked as spam or even outright rejected, leaving you wondering why legitimate messages aren't reaching their destination.
Understanding why SPF breaks during forwarding, especially when Gmail is the final recipient, is crucial for maintaining good email deliverability. It's a common headache for many users and administrators, but with a clear grasp of the underlying mechanisms, you can diagnose and address these issues effectively.
At its core, email forwarding involves one mail server receiving a message and then sending it on to another recipient address. When an email is initially sent, the sending server identifies itself and the sender's domain using various authentication protocols, including SPF. SPF works by verifying that the sending IP address is authorized by the domain listed in the email's Return-Path (also known as the MAIL FROM address).
The issue arises because when an email is forwarded, the forwarding server effectively becomes a new sender. This server's IP address might not be authorized in the original sender's SPF record. When the final recipient's mail server (e.g., Gmail) performs an SPF check, it sees the forwarding server's IP, which doesn't match the original domain's SPF record, leading to an SPF failure. This is why email forwarding often affects SPF validation.
To counteract this, some forwarding services implement Sender Rewriting Scheme (SRS). SRS rewrites the Return-Path address to reflect the forwarding server's domain, thereby ensuring the SPF check passes. However, if SRS is not used or is misconfigured, SPF will undoubtedly fail. This mechanism is fundamental to how SPF authentication works.
What is SPF?
SPF is an email authentication protocol that allows domain owners to publish a DNS record specifying which mail servers are authorized to send email on behalf of their domain. Receiving mail servers use this record to verify incoming mail. If an email originates from a server not listed in the SPF record, it may fail authentication.
Gmail's strict stance on authentication
Gmail has stringent email authentication policies. As a leading mailbox provider, Google actively checks SPF, DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance) to combat spam and phishing. When an email is forwarded to a Gmail account, Gmail's servers perform these checks.
If the forwarding server modifies the email headers in any way, it can break the DKIM signature. Furthermore, as discussed, if the forwarding server’s IP isn't authorized in the original domain's SPF record and SRS isn't correctly implemented, SPF will fail. Gmail interprets these authentication failures as a sign of potential spoofing or malicious activity, leading to rejection or placement in the spam folder. This is a common reason why emails get marked as spam when forwarded to Gmail.
While SPF directly authenticates the sending server's IP against the Return-Path domain, DKIM authenticates the message content and headers, and DMARC dictates what to do if either SPF or DKIM fails. All three work together, and a failure in one, especially SPF due to forwarding, can trigger a DMARC failure, which impacts overall deliverability.
SPF challenges
Identity mismatch: SPF checks the Return-Path domain against the connecting IP. When forwarded, the IP changes, causing SPF to break.
No SRS: Without Sender Rewriting Scheme, the original Return-Path remains, leading to SPF failures when forwarded.
DKIM considerations
Header modification: DKIM validates message headers and body. If a forwarding server alters these, the DKIM signature can break.
Domain alignment: Even with a valid DKIM, DMARC requires the DKIM signing domain to align with the From domain. Forwarding can complicate this alignment.
Why forwarded emails often fail SPF
One of the primary culprits behind SPF verification failures for forwarded emails is the original sender's SPF policy. If the sender's SPF record uses a -all mechanism, it specifies a hard fail for any mail server not explicitly listed as authorized. When such an email is forwarded through a server not included in the original SPF record, the recipient (like Gmail) will see this as a hard fail and often reject the message outright. This is a common reason why Gmail can reject emails, even if SPF sometimes seems to pass.
Another significant cause is the forwarding server itself. If the forwarding service does not implement SRS, or if its SRS implementation is faulty, SPF is bound to fail. Without SRS, the Return-Path remains that of the original sender, while the IP address used for forwarding is different, leading to an SPF mismatch. Some services might also break the DKIM signature by modifying message headers, which further exacerbates authentication issues, especially with DMARC policies in place.
Finally, complex SPF records with too many DNS lookups can also lead to failures, sometimes resulting in a TempError. SPF has a limit of 10 DNS lookups. If an SPF record includes multiple include or a mechanisms that require additional lookups, it can exceed this limit. This often leads to a DMARC report showing SPF TempError statuses, which Gmail may treat as a soft fail or even a full fail. Even if the forwarding server is compliant, a misconfigured or overly complex SPF record on the original sender's domain can doom the email.
Example SPF record causing hard fails for non-authorized sendersDNS
When facing SPF failures for forwarded emails, the first step is to examine the email headers of the rejected message. These headers contain a wealth of information about the email's journey and authentication results. Look for Received-SPF, Authentication-Results, and ARC-Authentication-Results headers. They will tell you which server performed the SPF check, the result (pass, fail, softfail, neutral), and the domain against which the check was made. This helps determine whether the failure occurred at the forwarding server or at Gmail. You can also use online tools to check authentication results in headers.
For senders whose emails are being forwarded, it's best practice to use a ~all (softfail) mechanism in their SPF record instead of -all (hard fail) if forwarding is expected. A softfail indicates that the sender's domain doesn't explicitly authorize the sending IP, but it doesn't forbid it either, giving the recipient server more flexibility. For forwarding services, ensuring proper SRS implementation is critical. This rewrites the Return-Path to include the forwarding domain, allowing SPF to pass. This helps prevent SPF authentication fluctuations in Google Postmaster Tools.
If direct forwarding continues to cause issues, consider alternative solutions. Many modern email services offer robust inbox management features that negate the need for simple forwarding, such as alias addresses or shared inboxes. For domain owners, implementing a DMARC monitoring service can provide insights into how your emails are being authenticated across various mailbox providers, including Gmail, and help identify persistent SPF or DKIM failures.
SPF result
Meaning
Impact on deliverability
Pass
The sending IP is authorized by the SPF record of the Return-Path domain.
Positive authentication, usually good for inbox placement.
Fail (-all)
The sending IP is explicitly NOT authorized. Occurs if the original sender uses -all.
High likelihood of rejection or placement in the spam folder.
Softfail (~all)
The sending IP is not explicitly authorized, but not forbidden either. Occurs if the original sender uses ~all.
Email may be accepted but could still land in spam. Less severe than a hard fail.
Neutral (?all)
The sender's SPF record indicates no strong policy regarding the sending IP.
Receiving servers will rely on other authentication checks. Not ideal, but not a direct failure.
TempError (or None)
A transient error during SPF lookup, or no SPF record found for the domain.
Email is typically treated as softfail; it may be accepted but is more likely to go to spam.
Conclusion
SPF verification failures for emails forwarded to Gmail are a common challenge rooted in how email authentication protocols interact with mail forwarding mechanisms. The core problem is that SPF checks the sending IP against the original sender's domain, and a forwarding server changes that IP. If the original SPF record has a strict hard fail policy (using -all) and the forwarding service doesn't properly implement SRS, Gmail is highly likely to reject the email, or send it to the spam folder. Understanding this interaction is the first step to resolving the issue.
To mitigate these failures, senders should consider using a ~all policy and forwarding services must ensure robust SRS implementation. By actively monitoring email authentication results and adapting strategies, you can significantly improve the deliverability of forwarded emails to Gmail and other strict mailbox providers. This proactive approach helps in navigating the complexities of email security and ensuring your messages reach their intended recipients.
Views from the trenches
Best practices
Ensure your SPF record uses a softfail (via ~all) rather than a hard fail (via -all) if you anticipate your emails being forwarded by recipients.
Always check the full email headers, especially `Received-SPF` and `Authentication-Results`, to pinpoint where SPF failures occur in the forwarding chain.
Verify that your email forwarding service uses Sender Rewriting Scheme (SRS) to prevent SPF breaks by rewriting the Return-Path.
Implement DMARC for your domain to gain visibility into authentication failures, including those caused by forwarding, via aggregate reports.
Regularly review your DMARC reports to identify patterns of SPF failures related to forwarded messages and adjust your policies or forwarding setup as needed.
Common pitfalls
Misinterpreting bounce messages from Google as direct Gmail SPF rejections, when the rejection actually came from an upstream forwarder.
Using a strict SPF hard fail policy (-all) on your domain, which explicitly tells recipient servers not to accept mail from unauthorized IPs, including legitimate forwarders.
Relying on email forwarding services that do not properly implement SRS, leading to SPF alignment failures.
Ignoring the impact of email forwarding on DKIM signatures, which can also break if headers or content are modified by the forwarding server.
Not understanding that Gmail's 'The domain' SPF error often refers to the original sender's domain, even if the bounce is relayed by Google.
Expert tips
If frequent SPF failures occur due to forwarding, consider educating your users or opting for direct email delivery to mailboxes instead of relying on forwarding.
For critical communications, implement robust DMARC policies at quarantine or reject to enforce authentication, but be mindful of how this might impact legitimate forwarded mail if not managed carefully.
Analyze DMARC aggregate reports closely for insights into how various mailbox providers, including Gmail, are handling your authenticated and forwarded emails.
Use email header analysis tools to quickly debug authentication issues, as they can reveal the exact SPF status and the server responsible for the failure.
Communicate with your email forwarding service provider to understand their authentication handling, especially SRS implementation, and troubleshoot any discrepancies.
Expert view
Expert from Email Geeks says that the domain's SPF record, if it uses a hard fail policy, is explicitly telling Google not to allow mail to be forwarded, and Google is simply adhering to this instruction.
May 29, 2025 - Email Geeks
Expert view
Expert from Email Geeks says that if a forwarding service attempts to send mail to a Google address, and they are breaking the DKIM signature while not using SRS, the mail is likely to fail due to a hard fail SPF record from the original sender.
