Why are emails being marked as spam when they're forwarded to Gmail addresses?
Matthew Whittaker
Co-founder & CTO, Suped
Published 11 Jun 2025
Updated 19 Aug 2025
11 min read
It can be frustrating when emails you send are marked as spam, especially when they are simply being forwarded to Gmail addresses. This issue often stems not from the original sending practices, but from how email forwarding interacts with the stringent authentication and anti-spam measures employed by Gmail. Understanding these technical nuances is key to diagnosing and mitigating the problem.
When an email is forwarded, it essentially goes through another server before reaching its final destination. This intermediate hop can disrupt the email's original authentication, making it appear suspicious to sophisticated spam filters. Gmail applies a complex algorithm to assess incoming messages, and discrepancies in forwarded emails often trigger spam flags.
The core of the problem often lies in the interaction between email forwarding and established authentication protocols such as Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting, and Conformance (DMARC). These protocols are designed to verify the sender's identity and prevent spoofing. When an email is forwarded, the journey from sender to recipient becomes more complicated, and the original authentication can easily break.
This guide will explore the primary reasons why forwarded emails end up in the spam folder (or junk folder), delving into the technicalities of email authentication, the role of sender reputation, and how content can be affected. By understanding these factors, you can better troubleshoot and address deliverability issues for forwarded messages.
Email authentication failures
SPF (Sender Policy Framework) is a DNS record that lists which IP addresses are authorized to send email on behalf of a domain. When an email is directly sent, the receiving server checks the SPF record of the sending domain against the IP address of the server that sent the email. If they match, SPF passes. However, with forwarding, the email is re-sent by the forwarding server.
The IP address of the forwarding server is typically not listed in the original sender's SPF record. This mismatch causes the SPF check to fail. Since Gmail places high importance on email authentication, an SPF failure can be a strong signal for marking an email as spam or putting it on a blacklist (or blocklist). This is a common challenge for email aliases. To learn more about this, check out our guide on what causes SPF verification failures for forwarded emails.
DKIM (DomainKeys Identified Mail) provides a cryptographic signature that verifies the sender's domain and ensures the email content has not been tampered with in transit. The DKIM signature is generated based on specific parts of the email, including headers and the body. While DKIM is more resilient to forwarding than SPF, it can still break.
If the forwarding server modifies the email, even subtly, by adding or changing headers that are part of the DKIM signature, the signature can become invalid. Gmail checks this signature rigorously, and a failed DKIM check significantly increases the likelihood of the email being marked as spam. For more on these critical protocols, see our simple guide to DMARC, SPF, and DKIM.
Sender reputation and user feedback
Even if your original emails have a strong sender reputation, the act of forwarding can muddy the waters. Gmail's filters evaluate not only the original sender but also the reputation of the intermediate forwarding server. If that server has a poor reputation, perhaps due to other users forwarding spam through it, it can negatively impact your legitimate forwarded mail. This is why it's so important to track your sender reputation using tools like Google Postmaster Tools or blocklist monitoring.
User feedback also plays a significant role. If a recipient of a forwarded email mistakenly marks it as spam, Gmail learns from this behavior. A high rate of spam complaints, even for forwarded messages, can signal to Gmail that emails from the original domain, regardless of forwarding, are undesirable. This can lead to a damaged sender reputation. Recipients must mark emails as "Not Spam" to reverse this.
Maintaining a healthy sender reputation, even when relying on forwarding, requires diligence. Poor engagement metrics, such as low open rates or high unsubscribe rates, can further signal to Gmail that your emails are not valuable to recipients, irrespective of whether they were directly sent or forwarded. This comprehensive view of your email health is crucial for avoiding the spam folder.
Ensuring optimal sender reputation
To improve overall email deliverability to Gmail, whether directly sent or forwarded, focus on the following:
Monitor blocklists: Regularly check if your domain or IP is listed on any email blocklists or blacklists, as this severely impacts deliverability.
Maintain engagement: Encourage recipients to open and reply to your emails to signal positive engagement to Gmail.
Clean your lists: Remove inactive or invalid addresses to avoid bounces and spam traps.
Content and formatting considerations
The content of an email itself can contribute to it being marked as spam, and this risk can sometimes be amplified when an email is forwarded. Forwarding mechanisms can occasionally alter the email's formatting, add extra headers, or even modify links, which can trigger spam filters.
Spam filters, including Gmail's, look for suspicious keywords, excessive use of capitalization or exclamation marks, and problematic links (such as shortened URLs). If a forwarded email inadvertently introduces any of these elements, or if the forwarding process breaks legitimate links, it can raise red flags. Avoid using link shorteners in emails.
Additionally, a Gmail recipient might have a personal filter that automatically marks forwarded messages from certain senders as spam, or they might have previously marked an email from the original sender as spam, training their Gmail account to do so. This personal filtering can override general deliverability best practices for specific users. This can be especially frustrating if you're trying to send marketing emails or newsletters that are legitimate, as we discuss in our articles on why newsletters go to spam and marketing emails going to Gmail spam.
Before forwarding
The email originates directly from the sender's mail server.
SPF: The IP address of the sending server aligns with the domain's SPF record, resulting in a pass.
DKIM: The email content and relevant headers are intact, allowing the DKIM signature to verify successfully.
DMARC:SPF or DKIM align with the From: domain, passing authentication.
After forwarding
The email is re-sent by an intermediate forwarding server.
SPF: The forwarding server's IP often does not match the original domain's SPF record, leading to a softfail or fail.
DKIM:Forwarding servers (like Microsoft 365 or Google Workspace) can add headers or slightly modify the message, causing the DKIM signature to break.
DMARC: With SPF or DKIM failures and lack of alignment, DMARC authentication fails, leading to potential spam categorization or rejection, especially with a strict policy (p=quarantine or p=reject).
Volume and thresholds
One common question that arises is how Gmail's spam filters react to the sheer volume of forwarded emails. While a few forwarded messages are unlikely to cause issues, a high volume of emails forwarded from an email alias, especially those with authentication failures, can contribute to a negative sender reputation for the original domain. This means that even if you don't directly send to Gmail users, your domain's health with Gmail is still being built based on forwarded traffic. We often encounter this scenario, where a client's email performance suffers because of legitimate emails being passed through less secure or incorrectly configured forwarding services. This can result in delivery delays and spam placement issues for emails sent to Gmail.
The exact threshold at which Gmail begins to penalize domains for forwarded emails with authentication failures is not publicly disclosed. However, it is safe to assume that a consistent pattern of SPF or DKIM breakage, especially coupled with other negative signals like spam complaints or low engagement, will contribute to Gmail marking these messages as spam (or blacklisting them). You might notice your Gmail emails going to spam all of a sudden. This is explored further in our article Why are my emails suddenly going to spam in Gmail?.
While you cannot directly control how an email is forwarded by a third-party server, implementing a robust DMARC policy with alignment for SPF and DKIM can provide some level of control. A DMARC record with a policy of p=none allows you to monitor authentication failures, including those from forwarded mail, without immediately impacting deliverability. This insight can help you identify if significant volumes of your emails are being affected by forwarding issues and inform your strategy. For an example of this, check out our guide on DMARC p=none policy examples.
Troubleshooting and solutions
When forwarded emails consistently land in the spam folder, it's crucial to investigate the root cause. This involves checking your email authentication records, analyzing email headers of forwarded messages, and understanding the behavior of the forwarding server.
First, ensure your original email setup for SPF, DKIM, and DMARC is correctly configured and passes all checks when sent directly. Tools that provide an email deliverability test can help you confirm this. Next, obtain a forwarded email that ended up in spam and examine its full headers. Look for Authentication-Results headers to see where SPF, DKIM, and DMARC might have failed after forwarding. This will pinpoint the exact reason why Gmail marked them as spam.
If the issue is primarily with authentication failures due to forwarding, communicate with the administrators of the forwarding service (if possible) to see if they can configure their systems to maintain authentication. Some forwarding services employ SRS (Sender Rewriting Scheme) to address SPF breakage, but this is not universally applied. For instances where emails are suddenly rejected by Gmailafter forwarding, analyzing the bounces can also provide valuable insights.
Remember, the goal is to reduce any signals that Gmail might interpret as spammy. This means ensuring robust authentication, maintaining a positive sender reputation (even through forwarded channels), and educating recipients to mark legitimate emails as Not Spam. If you face persistent issues, consider alternative delivery methods that do not rely on forwarding for critical communications, such as direct email delivery via an SMTP server.
Views from the trenches
Best practices
Always ensure SPF, DKIM, and DMARC records are correctly set up and align for your sending domain, especially if emails are forwarded through an alias.
Educate recipients on how to mark legitimate forwarded emails as 'Not Spam' in their Gmail accounts.
Monitor your domain's reputation using tools like Google Postmaster Tools for any signs of degradation.
Common pitfalls
Assuming that because you don't directly email Gmail users, your sender reputation with Gmail is irrelevant, overlooking forwarded traffic.
Not regularly monitoring DMARC reports, which can reveal widespread authentication failures due to forwarding.
Using generic email forwarding services that do not preserve email authentication, leading to SPF or DKIM breakage.
Expert tips
Sender rewriting scheme (SRS) is crucial for forwarding services to prevent SPF failures. If your forwarding service doesn't use it, consider a different one.
Gmail's anti-spam algorithms are dynamic. Consistent authentication failures, even from forwarded mail, will eventually lead to deliverability issues.
A high volume of forwarded emails that break authentication can signal malicious activity to mailbox providers like Gmail, even if the content is benign.
Marketer view
Marketer from Email Geeks says that relying solely on GPT to analyze forwarded email issues might be pointless if the data doesn't differentiate between directly sent and forwarded emails.
2024-11-08 - Email Geeks
Expert view
Expert from Email Geeks says that not mailing specific domains like Gmail can be a red flag for abusive activities, unless there is a very good reason, which is rare.
2024-11-09 - Email Geeks
Navigating forwarded email deliverability
Dealing with forwarded emails being marked as spam by Gmail is a nuanced challenge in email deliverability. It's often not a reflection of your initial sending practices, but rather the intricacies of how email forwarding interacts with robust anti-spam systems.
The primary culprits are typically SPF authentication failures, potential DKIM breakage, and the impact of the forwarding server's or your domain's accumulating sender reputation (or blacklist status). By understanding these underlying issues, monitoring your email's journey, and advocating for proper email authentication, you can significantly improve the chances of your forwarded messages reaching the inbox.
Gmail. Understanding these technical nuances is key to diagnosing and mitigating the problem.
Gmail, whether directly sent or forwarded, focus on the following:
Forwarding servers (like Microsoft 365 or Google Workspace) can add headers or slightly modify the message, causing the DKIM signature to break.
