Summary
Email forwarding commonly results in DMARC authentication failures, primarily because the forwarding server becomes the apparent sender, disrupting the delicate balance required for SPF and DKIM validation. When an email is forwarded, the recipient's mail server sees the forwarding server's IP address, not the original sender's. This typically causes SPF to fail, as the forwarding server's IP is not authorized by the original domain's SPF record. While DKIM is generally more robust, modifications to message content or headers by the forwarding service can invalidate the DKIM signature, leading to its failure as well. Since DMARC requires at least one of these authentication methods to pass and align, the failure of both leads to DMARC failure, often resulting in rejection or quarantine by receiving mailbox providers like Gmail. The unusual scenario of multiple accounts from the same provider consistently forwarding to a single Gmail address suggests potential underlying issues, ranging from deliberate user-set forwarding or catch-all policies to more concerning possibilities such as mailbox compromise or stealing.
Key findings
- SPF Breaks on Forwarding: Email forwarding almost universally breaks SPF authentication because the forwarding server's IP address, not the original sender's, is presented to the final recipient, and this IP is not authorized by the original domain's SPF record.
- DKIM Vulnerable to Modification: While more resilient than SPF, DKIM can also fail if the forwarding service alters the email's content or headers, invalidating the cryptographic signature and leading to a DMARC failure.
- DMARC Relies on Alignment: DMARC mandates that the 'From' header domain aligns with an authenticated SPF or DKIM domain. When both SPF and DKIM fail due to forwarding, DMARC fails, often causing rejection, especially under strict DMARC policies.
- Forwarder Becomes Apparent Sender: The core issue is that the forwarding server acts as the sender in the final leg of delivery, meaning authentication checks are performed against its IP and any modifications it makes, rather than the original sender's.
- Unusual Forwarding Patterns: Multiple accounts from the same provider forwarding to a single Gmail address is an atypical pattern that could indicate user-configured forwarding, a domain's catch-all policy, or potentially a privacy violation like mailbox compromise.
Key considerations
- Review DMARC Reports: Regularly examining DMARC reports can help identify the source IPs of forwarded emails and detect suspicious forwarding patterns, such as multiple accounts routing to an unexpected single destination.
- Contact Mailbox Provider: If unusual forwarding behavior or potential mailbox compromise is suspected, especially when multiple accounts are involved, contacting the affected mailbox provider is a crucial step for investigation.
- Forwarder Implementation Varies: The extent to which DMARC breaks depends on the specific forwarding service; some may attempt to preserve DKIM, while others modify content more aggressively.
- Understand DMARC Policy Impact: A domain's DMARC policy (e.g., 'p=reject' or 'p=quarantine') determines the consequence of forwarding-induced DMARC failures, potentially leading to non-delivery of legitimate forwarded emails.
- Distinguish Bounce Sources: If an email to a forwarded address bounces, identify whether the bounce originated from Gmail or the initial mailbox provider, as this can offer clues about the cause of the DMARC failure.
What email marketers say
10 marketer opinions
Email forwarding frequently triggers DMARC authentication failures, especially when messages are routed to a single Gmail account. This occurs primarily because the forwarding server assumes the role of the apparent sender in the final leg of delivery. Consequently, SPF authentication fails as the forwarding server's IP address, not the original sender's, is presented to the recipient, and this IP is typically unauthorized by the original domain's SPF record. Furthermore, if the forwarding service modifies the email's content or headers, the DKIM signature can be invalidated, leading to a DKIM failure. With both SPF and DKIM failing authentication, and DMARC requiring at least one to pass and align, the forwarded email will not satisfy the DMARC policy and is often rejected or quarantined by receiving mailbox providers such as Gmail. The specific scenario of varied accounts from the same provider consistently forwarding to a single Gmail address is unusual and warrants investigation, as it could indicate a legitimate user configuration, a domain-wide catch-all policy, or more concerning issues like mailbox compromise.
Key opinions
- SPF Fails on Forwarding: The primary reason DMARC fails with forwarded emails is that SPF validation occurs against the IP address of the forwarding server, not the original sender. Since the forwarder's IP is not authorized by the original domain's SPF record, SPF checks inevitably fail.
- DKIM Vulnerability: While DKIM is generally more robust than SPF, it can still break if the forwarding server alters the email's headers or content. Any modification to the message invalidates the cryptographic signature, leading to a DKIM authentication failure and, consequently, a DMARC failure.
- DMARC Alignment Required: DMARC requires at least one of SPF or DKIM to pass authentication and align with the 'From' domain. When both fail due to forwarding, the DMARC policy instructs the receiving mailbox provider, like Gmail, to reject or quarantine the email.
- Forwarder as Apparent Sender: Email forwarding effectively changes the apparent sender of the email from the original source to the forwarding server. This shift means that authentication checks are performed against the forwarding server's credentials and actions, leading to misalignments with the original sender's DMARC policy.
- Mailbox Provider Definition: For clarity in email deliverability discussions, 'mailbox provider' is the appropriate term for an entity that provides customer mailboxes, distinguishing it from an 'ESP' that primarily handles bulk sending for marketers.
- DMARC Often Breaks by Design: Forwarding processes are frequently designed in a way that inherently breaks DMARC, particularly SPF, because they were not initially conceived with DMARC's strict authentication and alignment requirements in mind. This is a common and anticipated behavior in the email ecosystem.
Key considerations
- Analyze DMARC Reports: For unusual forwarding patterns, such as multiple accounts routing to a single Gmail address, DMARC reports are crucial for identifying the source IPs of the forwarding servers and confirming authentication failures. These reports can provide the initial clues needed for further investigation.
- Consult Mailbox Provider: If DMARC reports indicate suspicious forwarding activity, particularly across varied accounts from the same provider to one Gmail address, contacting the mailbox provider for the original accounts is advisable. This could help uncover issues like unintended catch-all policies or potential account compromises.
- Forwarder Variability: It's important to understand that DMARC outcomes depend on the specific forwarding service. While SPF almost always breaks, some forwarders might attempt to preserve DKIM, potentially allowing DMARC to pass if DKIM remains intact and aligned. Others are more aggressive with content changes, leading to DKIM failure.
- Identify Malicious Activity: The consistent forwarding of varied accounts from one provider to a single Gmail address is an atypical signal. It warrants investigation to rule out malicious activity, such as mailbox stealing, or to confirm legitimate but unusual configurations like a domain-wide catch-all to a single monitoring inbox.
Marketer view
Marketer from Email Geeks explains that DMARC often breaks by design when emails are forwarded. He clarifies that an 'ESP' providing customer mailboxes is typically referred to as a 'mailbox provider'. He also suggests that tagging or changes to the message content during the forwarding process can break DKIM, leading to DMARC failures. He agrees that if varied accounts from the same provider consistently forward to one Gmail address, it indicates something unusual, possibly mailbox stealing, and advises checking DMARC reports for source IPs and contacting the mailbox provider.
17 Dec 2022 - Email Geeks
Marketer view
Marketer from Email Geeks explains that DMARC behavior depends on the forwarder, noting that in many cases, forwarding breaks SPF but DKIM may survive, which means DMARC can still pass authentication.
27 Mar 2022 - Email Geeks
What the experts say
3 expert opinions
The phenomenon of mailbox providers routing emails to a single Gmail account, leading to DMARC failures, involves a blend of technical authentication breakdowns and unusual forwarding scenarios. Fundamentally, DMARC often fails because standard email forwarding mechanisms alter crucial authentication components. Specifically, SPF authentication relies on the Return-Path, which shifts to the forwarding server's domain, causing it to no longer align with the original sender's domain. While DKIM is more resilient, forwarding services can still invalidate its signature through modifications to message content or headers. Since DMARC mandates that either SPF or DKIM pass authentication and align with the 'From' header, such failures result in emails being rejected or quarantined, particularly by services like Gmail with strict DMARC enforcement. The specific pattern of multiple accounts from the same provider consistently forwarding to one Gmail address is atypical, suggesting possibilities ranging from individual user configuration or a domain-wide catch-all policy to more concerning privacy violations like mailbox compromise or stealing.
Key opinions
- SPF Failure Core Cause: Email forwarding inherently breaks SPF authentication because the Return-Path domain changes to the forwarding server's, causing a mismatch with the original sender's domain in the 'From' header.
- DKIM Vulnerability: Although more robust, DKIM can also fail if the forwarding service modifies the email's content or headers, invalidating its cryptographic signature.
- DMARC Alignment Rule: DMARC requires either SPF or DKIM to pass authentication and align with the 'From' domain. When both fail due to forwarding, DMARC policies lead to rejection or quarantine, especially by strict receivers like Gmail.
- Multiple Forwarding Theories: When varied accounts from a single provider forward to one Gmail address, potential causes include individual user setup, a small domain's catch-all policy, or more serious issues like mailbox compromise or stealing.
- Bounce Source Indicator: If an email bounces after forwarding, the bounce message typically originates from the initial mailbox provider, not Gmail, which helps identify where the DMARC failure or forwarding issue began.
- Compromise Signal: The consistent occurrence of different accounts from the same mailbox provider forwarding to a single Gmail account over time is a strong indicator of potential mailbox compromise or stealing.
Key considerations
- Analyze Forwarding Patterns: Investigate consistent, unusual forwarding patterns, such as multiple accounts routing to a single destination, to identify potential misconfigurations or malicious activity.
- Review DMARC Reports: DMARC reports are essential for understanding the authentication failures caused by forwarding and for identifying the source IP addresses of the forwarding servers.
- Verify Bounce Origins: Distinguishing whether a bounce comes from the initial mailbox provider or the final Gmail recipient helps pinpoint the exact point of failure in the delivery chain.
- Engage Mailbox Providers: If suspicious or domain-wide forwarding to a single address is observed, contacting the affected mailbox provider is crucial for investigating the cause, such as a catch-all policy or security breach.
Expert view
Expert from Email Geeks shares several theories regarding why multiple email addresses from a mailbox provider might forward to a single Gmail account, causing DMARC failures. She suggests it could be an individual account owner setting up forwarding, a small domain with a catch-all policy autoforwarding all mail to Gmail, or a more serious privacy violation such as mailbox stealing by the mailbox provider or another party. She clarifies that Gmail doesn't typically send async bounces, implying the bounce is likely coming from the mailbox provider. When Christopher confirms different accounts from the same provider exhibit this behavior over time, she leans towards the possibility of mailbox compromise or stealing.
6 Dec 2022 - Email Geeks
Expert view
Expert from Spam Resource explains that when an email is forwarded by a mailbox provider, the original sender's domain in the 'From' header does not match the IP address of the forwarding server. This causes SPF authentication to fail. Since DMARC requires either SPF or DKIM to pass and align, the SPF failure (and potential DKIM issues with forwarding services) leads to a DMARC failure, which can cause the email to be rejected or quarantined, especially by providers like Gmail with strict DMARC enforcement.
15 May 2025 - Spam Resource
What the documentation says
4 technical articles
Email forwarding frequently causes DMARC authentication failures, especially when messages are routed to recipients at major mailbox providers like Gmail. This issue arises because the forwarding server interposes itself in the delivery path, effectively becoming the apparent sender. Consequently, SPF checks often fail since the forwarding server's IP address is not authorized by the original sender's SPF record. Moreover, if the forwarding process modifies the email's content or headers, the DKIM signature becomes invalid, leading to a DKIM failure. As DMARC policies mandate that the 'From' header domain aligns with an authenticated SPF or DKIM domain, the simultaneous failure of both authentication methods means the forwarded email will not pass DMARC validation, resulting in its rejection or quarantine by the final recipient's mail system.
Key findings
- SPF Disruption: SPF authentication is typically broken during email forwarding because the forwarding server's IP address, not the original sender's, is presented to the final recipient, and this IP is not authorized by the original domain's SPF record.
- DKIM Invalidation Risk: While generally robust, DKIM signatures can be invalidated if the forwarding service alters the email's content or headers, leading to an authentication failure and subsequent DMARC failure.
- DMARC Alignment Failure: DMARC explicitly requires the 'From' header domain to align with a domain successfully authenticated via SPF or DKIM. When both fail due to forwarding, DMARC policies are violated, leading to rejection or quarantine.
- Forwarder as New Source: The act of forwarding introduces an intermediary server that acts as the sending source for the final delivery, fundamentally changing the context for authentication checks against the original domain.
- Mailbox Provider Enforcement: Services like Gmail rigorously perform DMARC checks, and emails that fail DMARC due to forwarding are often rejected or sent to spam folders, even if they originated from a legitimate sender.
Key considerations
- Inherent DMARC Challenge: Email forwarding inherently poses a challenge for DMARC, as its authentication mechanisms were not designed to fully account for the retransmission and potential modification of emails by intermediary servers.
- Sender Awareness Critical: Senders should recognize that their emails may fail DMARC when forwarded, potentially impacting deliverability to recipients who rely on such forwarding services for their mail flow.
- Recipient Impact: Recipients using forwarding services may experience non-delivery or spam classification of legitimate emails from DMARC-protected domains due to the authentication failures during the forwarding process.
- Forwarder Specifics: The precise impact on DKIM can vary depending on whether the forwarding service modifies the email body or headers, while SPF authentication is almost universally broken by forwarding.
Technical article
Documentation from M3AAWG.org explains that email forwarding often breaks DMARC authentication because the forwarder's server, not the original sender's, is the one sending the email to the final recipient. This change in the 'sending' server can cause SPF to fail as the forwarder's IP is not authorized by the original domain's SPF record, and DKIM may also fail if the message content or headers are modified during forwarding, leading to DMARC failure.
7 Feb 2024 - M3AAWG.org
Technical article
Documentation from RFC Editor (RFC 7489) explains that DMARC relies on the 'From' header domain aligning with either the SPF or DKIM authenticated domain. Email forwarding often introduces a new 'relay host' (the forwarding server) whose IP address is not authorized by the original sender's SPF record, causing SPF failure. Additionally, modifications to message content or headers by the forwarding agent can invalidate the DKIM signature, leading to DMARC failure because neither SPF nor DKIM passes alignment.
2 Nov 2022 - RFC Editor
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