Why does Gmail say it cannot verify the sender and mark the email as spam?
Matthew Whittaker
Co-founder & CTO, Suped
Published 28 Jun 2025
Updated 19 Aug 2025
8 min read
Seeing a message from Gmail stating it cannot verify the sender or that an email might be spam can be unsettling, especially when you know the email is legitimate. This warning often appears as a grayed-out icon next to the sender's name, accompanied by a note like The Webby Mail cannot verify that this sender is not a spammer. It's a clear signal that something is amiss with your email's authentication or overall deliverability health.
This isn't just an annoyance, it directly impacts how your recipients perceive your messages and whether they even see them in their inboxes. When Google's sophisticated spam filters flag an email, it's often due to unverified sender information, leading to messages being routed to the spam folder or displaying prominent warnings. Understanding the root causes is the first step toward a fix.
Understanding Gmail's verification process
Email authentication is the cornerstone of modern email security and deliverability. Gmail and other mailbox providers rely on a set of protocols to confirm that an email truly originates from the domain it claims to be from. Without these checks, it's easy for malicious actors to impersonate legitimate senders, leading to phishing and spam. When these checks fail, Gmail displays the 'cannot verify sender' warning and increases the likelihood of the email being marked as spam.
The primary authentication methods are Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting & Conformance (DMARC). Gmail's spam filtering algorithms heavily weigh the results of these checks. An email that fails one or more of these authentications raises a red flag, signaling potential spoofing or unauthorized sending. This is why you might see messages like Gmail couldn't verify that domain actually sent this message.
Implementing a robust email authentication setup is no longer optional; it's a critical requirement for anyone sending email, whether it's personal correspondence or marketing campaigns. Without it, your emails are essentially untrustworthy in the eyes of mailbox providers, directly impacting your email deliverability.
Common causes of verification failure
Several factors can lead to Gmail displaying an unverified sender warning or marking your emails as spam (also called a blocklist or blacklist entry). The most common culprits revolve around misconfigured or missing DNS records for authentication. An improperly set up SPF record, such as one with syntax errors or that doesn't include all authorized sending IPs, is a frequent cause. For instance, a common mistake is having multiple SPF records or including unexpected characters, which can break the record and prevent proper verification.
Similarly, a missing or invalid DKIM signature can lead to verification issues. DKIM provides a cryptographic signature that verifies the email content hasn't been tampered with in transit. Without it, Gmail can't confirm the email's integrity. The absence of a DMARC policy, or having one set to p=none without proper alignment, also signals to mailbox providers that you're not fully authenticating your emails. This could also be why Gmail might flag an email as suspicious due to low sender reputation.
Furthermore, scenarios like email forwarding can introduce complications. When an email is forwarded, its headers can be altered, potentially breaking SPF or DKIM alignment, leading to an authentication failure at the final destination. This is why legitimate forwarded emails sometimes end up in spam folders, or cause SPF verification failures for emails forwarded to Gmail. Another often overlooked aspect is your IP address or domain ending up on a email blocklist (or blacklist), which significantly degrades your sender reputation. Even if your authentication is technically correct, a poor reputation can override it, leading to messages being blocked or spam filtered.
Unverified and spam consequences
Emails that are not properly verified or are marked as spam face several detrimental outcomes:
Low deliverability: Messages are frequently diverted to the spam folder, or even rejected, regardless of content.
Reduced engagement: Recipients are less likely to open emails with warnings, impacting open and click-through rates.
Brand erosion: Consistent warnings can lead recipients to distrust your brand and mark your emails as junk.
Steps to fix unverified sender warnings
The good news is that most unverified sender issues are fixable by addressing your email authentication. Start by inspecting your DNS records for SPF, DKIM, and DMARC. Tools like AboutMyEmail or online checkers can help identify misconfigurations. Ensure your SPF record includes all legitimate sending sources, your DKIM records are correctly published, and your DMARC policy is actively monitoring or enforcing authentication.
If you're using a third-party email service provider (ESP), ensure you've added their required SPF include mechanisms and DKIM CNAMEs to your DNS. It's also crucial to monitor your DMARC reports, which provide valuable insights into authentication failures and potential spoofing attempts. Gradually move your DMARC policy from p=none to p=quarantine or p=reject as you gain confidence in your authentication setup.
For individual or internal Gmail accounts, ensure you're not forwarding emails in a way that breaks authentication, which can lead to alerts like Gmail marking emails as phishing. If you're using a custom domain with Gmail, ensure your DNS records are correctly configured for your Google Workspace setup. These steps are fundamental to preventing your emails from being flagged.
Before fix: unverified status
Problem: DNS records for SPF, DKIM, or DMARC are missing or incorrect.
Outcome:Gmail displays 'unverified sender' or 'cannot verify' warnings.
Deliverability impact: Emails often land in spam or junk folders.
Trust: Recipients lose trust in the sender, perceiving emails as potentially dangerous.
After fix: verified delivery
Solution: Properly configured and aligned SPF, DKIM, and DMARC records.
Result:Gmail verifies the sender, often showing a checkmark or trusted status.
Deliverability improvement: Emails consistently reach the primary inbox.
Enhanced credibility: Recipients are more likely to open and engage with emails.
Maintaining a strong sender reputation
Beyond technical authentication, maintaining a strong sender reputation is paramount. Even with perfect SPF, DKIM, and DMARC, a poor reputation can lead to Gmail marking your emails as spam. Mailbox providers track various signals, including complaint rates, bounce rates, spam trap hits, and user engagement (opens, clicks, replies). Consistently sending unwanted emails, even if authenticated, will damage your domain's standing.
To improve or maintain your reputation, focus on sending relevant content to engaged subscribers. Avoid purchasing email lists, clean your lists regularly to remove inactive or invalid addresses, and make it easy for recipients to unsubscribe. Monitor your domain reputation through tools like Google Postmaster Tools. This proactive approach helps prevent your emails from being incorrectly classified as unverified or spam.
Consider implementing Brand Indicators for Message Identification (BIMI), which allows you to display your brand logo next to your sender name in supported inboxes. This visual verification, combined with strong DMARC enforcement, further boosts recipient trust and can contribute to better inbox placement. Even if your emails are fully authenticated, Outlook/Hotmail might still mark them as unverified if other reputation factors are poor.
Best practices for a healthy sender reputation
Segment your lists: Send targeted content to highly engaged subscribers.
Clean your lists regularly: Remove inactive users and hard bounces promptly.
Implement feedback loops: Receive notifications when recipients mark your emails as spam, and act on them.
Final thoughts on email verification
Understanding why Gmail might flag your emails as unverified or spam is crucial for effective email communication. It's almost always related to a fundamental issue with your email authentication, or your sender reputation, or both. Taking immediate action to audit and correct your SPF, DKIM, and DMARC records is the most effective first step.
Remember, email deliverability is an ongoing effort, not a one-time fix. Consistent monitoring of your authentication health, along with a focus on sending valuable content to an engaged audience, will ensure your messages bypass the spam folder and land where they belong: the inbox. This proactive approach will build trust with mailbox providers and your recipients, securing your email deliverability for the long term.
Gmail stating it cannot verify the sender or that an email might be spam can be unsettling, especially when you know the email is legitimate. This warning often appears as a grayed-out icon next to the sender's name, accompanied by a note like The Webby Mail cannot verify that this sender is not a spammer. It's a clear signal that something is amiss with your email's authentication or overall deliverability health.
Google's sophisticated spam filters flag an email, it's often due to unverified sender information, leading to messages being routed to the spam folder or displaying prominent warnings. Understanding the root causes is the first step toward a fix.
