Why is Gmail throwing errors and marking my emails as phishing?
Michael Ko
Co-founder & CEO, Suped
Published 26 Jun 2025
Updated 16 Aug 2025
7 min read
There are few things more frustrating in email marketing or communication than sending a seemingly legitimate email, only to have flag it with an alarming phishing warning. This can happen whether you're sending transactional emails, marketing newsletters, or even personal correspondence. When Gmail throws these errors and marks your emails as phishing, it severely impacts your deliverability and trustworthiness.
It's a clear signal that something about your email, or your sending infrastructure, has triggered sophisticated spam and phishing detection algorithms. These systems are designed to protect users from malicious content, but sometimes legitimate senders can inadvertently get caught in the crossfire.
Understanding the common reasons behind these warnings is the first step toward fixing them. We'll explore the main culprits, from technical authentication issues to suspicious content, and provide actionable steps to ensure your emails reach the inbox safely.
Sender reputation and blocklists
One of the most significant factors influencing whether your emails are flagged is your sender reputation. If your domain or IP address has a poor reputation, Gmail is more likely to view your emails with suspicion, even if they appear benign. A damaged reputation often stems from past sending practices, like high spam complaint rates or sending to invalid addresses, which can lead to being listed on email blocklists (also known as blacklists).
These blocklists (or blacklists) are databases of IP addresses and domains known to send spam or malicious emails. While your links might not be directly listed on a public DNSBL, Gmail maintains its own private, internal lists of questionable URLs and sender reputations. Even if you're not on a major public blacklist like Spamhaus, Google's filters can still flag you if they internally deem your links or domain suspicious.
Monitoring your sender reputation through tools like Google Postmaster Tools is crucial. If you see consistently low reputation scores or high spam rates, it’s a clear indication that you need to address underlying issues with your sending practices. For instance, sometimes a sender's entire IP range, such as those from cloud providers, can be associated with poor reputation if other users on that range engage in abusive sending.
Gmail's strictness
Gmail employs highly advanced AI and machine learning algorithms to detect potential phishing threats. These systems analyze countless data points, including sender reputation, authentication status, email content, link destinations, and user feedback. Even minor deviations from expected email behavior can trigger a warning.
Authentication failures
A fundamental reason for Gmail to mark emails as phishing is a failure in email authentication. Protocols like SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance) are designed to verify that an email truly came from the domain it claims to be from. If these checks fail, Gmail has strong reasons to suspect spoofing or phishing attempts.
For example, if your SPF record is missing or incorrectly configured, Gmail cannot verify that your sending server is authorized to send emails on behalf of your domain. Similarly, a broken DKIM signature indicates that the email may have been tampered with in transit. Both issues undermine trust.
DMARC builds upon SPF and DKIM, instructing receiving mail servers on how to handle emails that fail authentication. If your DMARC policy is set to a strict p=reject or p=quarantine and authentication fails, Gmail is obligated to follow that instruction, potentially marking your email as suspicious or rejecting it outright. New Google and Yahoo requirements for bulk senders make DMARC a mandatory step for better deliverability.
Beyond technical configurations, the actual content of your email and the links it contains are heavily scrutinized. Gmail is particularly wary of suspicious links that might lead to phishing sites or malware. Even if your links seem harmless to you, Google's automated systems might identify them as problematic if they've been associated with phishing in the past, or if they point to unindexed domains.
Common indicators include shortened URLs, links to login pages that aren't on your primary domain, or URLs that redirect multiple times. If Gmail detects any element of your email, including images or attachments, that resembles known phishing tactics, it will trigger a warning. This is why even legitimate companies sometimes see phishing warnings for linked login pages.
Another scenario occurs when an email contains a suspicious link that has been used to steal personal information. Gmail flags these emails to protect its users. Sometimes, a brand's website itself might have been compromised, hosting a phishing target page, which then causes legitimate emails linking to that site to be flagged as dangerous. This means the block isn't about your email sending, but about the compromised destination.
Urgent or threatening language, requests for personal information, too many exclamation marks, all caps, or suspicious attachments.
Links
Hyperlinks that don't match the displayed text, shortened URLs (e.g., bit.ly), links to unfamiliar or unindexed domains, multiple redirects.
Grammar and spelling
Numerous errors in grammar or spelling, which can be a tell-tale sign of a fraudulent email.
Unexpected contact
Receiving an email from a company or service you don't typically interact with, especially if it's about a critical issue.
User feedback and engagement
Ultimately, Gmail's filters learn from user behavior. If a significant number of recipients mark your emails as spam or phishing, it sends a strong negative signal to Google's systems. This can happen even if your emails are technically compliant and your content is legitimate, simply because recipients perceive them as unwanted or suspicious. Poor list hygiene, irrelevant content, or infrequent sending can contribute to this.
This highlights the importance of sending emails only to engaged subscribers who expect to hear from you. Regularly cleaning your email lists, removing inactive users, and providing clear unsubscribe options can help maintain a positive sender reputation and prevent recipients from manually marking your emails as spam or phishing. You can learn more about why your emails go to spam.
Furthermore, if Gmail detects unusual patterns, such as a sudden spike in sending volume or a drastic change in email content or sending behavior, it might trigger a temporary warning or even a block. This is part of their effort to mitigate sudden spam attacks or account compromises. Addressing these issues often requires a multifaceted approach, combining technical fixes with strategic content and list management.
Technical aspects to check
SPF record: Ensure it includes all authorized sending IP addresses and domains. Incorrect configurations can lead to authentication failures.
DKIM signature: Verify it's valid and applied to all outgoing emails. A missing or invalid signature raises suspicion.
DMARC policy: Implement a policy that aligns your SPF and DKIM, moving gradually to quarantine or reject after monitoring reports.
TLS encryption: Ensure your emails are sent over a secure, encrypted connection to prevent interception and tampering.
Content and engagement best practices
Clear links: Use full, descriptive URLs rather than shortened ones, and ensure they lead to the expected destination.
Relevant content: Avoid generic or spammy phrases, excessive capitalization, or too many exclamation marks.
List hygiene: Regularly clean your email lists to remove inactive or invalid addresses, reducing bounce and complaint rates.
Consistent sending: Maintain a consistent sending volume and pattern to build trust with Gmail's filters.
Views from the trenches
Best practices
Ensure all email authentication records (SPF, DKIM, DMARC) are correctly configured and aligned.
Regularly monitor your domain and IP reputation using Google Postmaster Tools and other monitoring services.
Scan all outbound links in your emails for potential malware or phishing flags before sending.
Maintain a clean and engaged email list, removing inactive or invalid addresses to reduce bounce and complaint rates.
Common pitfalls
Ignoring phishing warnings, assuming they are false positives, without investigating underlying causes.
Using generic link shortening services that may have been associated with malicious activity in the past.
Sending emails with inconsistent branding or sudden changes in sending patterns that can trigger filters.
Failing to respond to DMARC reports, missing critical insights into authentication failures and potential abuse.
Expert tips
Implement a DMARC policy at 'p=none' initially to gather reports and then move to 'p=quarantine' or 'p=reject'.
Perform regular email deliverability tests using an email deliverability tester to catch issues before campaigns go live.
Educate your team on common phishing indicators to prevent accidental inclusion of suspicious elements.
If using a shared IP, understand the sending practices of others sharing that IP, as their actions can impact your reputation.
Marketer view
Marketer from Email Geeks says a key reason for Gmail flagging emails is often the presence of a suspicious link that was previously used to steal personal information.
2022-11-05 - Email Geeks
Marketer view
Marketer from Email Geeks says if multiple ISPs are flagging messages as phishing, it strongly indicates that a phishing operation might be occurring using those links, potentially due to a compromised site.
2022-11-05 - Email Geeks
Fixing phishing warnings
Addressing Gmail's phishing warnings requires a systematic approach. Start by verifying all your email authentication records – SPF, DKIM, and DMARC – are correctly implemented and aligned. Use Google Postmaster Tools to monitor your domain and IP reputation regularly.
Next, thoroughly review your email content and all embedded links. Ensure all links resolve to expected, secure destinations. If your website has been compromised, promptly remove any malicious pages. Avoid using suspicious content elements, and make sure your branding is consistent and easily recognizable by recipients. Consider if your emails are being rejected for other reasons.
Finally, focus on maintaining a healthy sender reputation by sending relevant emails to engaged recipients. Clean your mailing lists regularly to minimize bounces and spam complaints. By proactively managing these aspects, you can significantly reduce the likelihood of Gmail marking your emails as phishing, ensuring your messages land where they belong: in the inbox.
There are few things more frustrating in email marketing or communication than sending a seemingly legitimate email, only to have flag it with an alarming phishing warning. This can happen whether you're sending transactional emails, marketing newsletters, or even personal correspondence. When Gmail throws these errors and marks your emails as phishing, it severely impacts your deliverability and trustworthiness.
It's a clear signal that something about your email, or your sending infrastructure, has triggered sophisticated spam and phishing detection algorithms. These systems are designed to protect users from malicious content, but sometimes legitimate senders can inadvertently get caught in the crossfire.
Spamhaus, Google's filters can still flag you if they internally deem your links or domain suspicious.
