Why are my emails marked as dangerous in Gmail when using microdata markup?
Michael Ko
Co-founder & CEO, Suped
Published 2 May 2025
Updated 16 Aug 2025
7 min read
It can be incredibly frustrating to find your legitimate emails landing in the spam folder, especially when you've gone the extra mile to enhance them with features like microdata markup. You might expect these additions to improve user experience and deliverability, but sometimes, they seem to do the opposite, triggering a This message seems dangerous warning in Gmail. It's a puzzling situation many email senders face, and it often leads to a deep dive into what might be going wrong with otherwise well-intentioned emails.
I've personally experienced the confusion when trying to implement structured data like microdata or JSON-LD for emails, only to see it negatively impact deliverability. It feels like a paradox: you add code designed to make your emails more functional and trustworthy, but mailboxes react by flagging them as suspicious. This usually points to a deeper issue than just the markup itself, often related to how Gmail perceives your sending domain or the overall email content.
My goal here is to help you understand why this happens and what steps you can take to prevent your emails from being incorrectly flagged as dangerous, allowing your valuable messages to reach the inbox safely.
The intricate role of email markup
Microdata, or structured data, is intended to enhance emails by providing context and enabling interactive features directly within the inbox. For example, you can use it to display flight confirmations, package tracking, or event details in a structured format. Gmail supports both JSON-LD and Microdata, aiming to make emails more useful and actionable for recipients.
However, with great power comes great responsibility. Structured data, by its nature, provides explicit signals about the content and purpose of an email. If these signals are inconsistent with Gmail's expectations, or if they are used in a way that mimics phishing attempts or other malicious activities, they can inadvertently trigger security warnings. It's a delicate balance between enhancing user experience and maintaining trust and security.
One critical aspect often overlooked is the sender's registration status. For certain advanced features and consistent rendering of markup, Google requires senders to be registered and whitelisted. Without proper registration, even perfectly valid microdata can be seen as suspicious, leading to emails being marked as dangerous. This is often the case for senders who implemented markup years ago before formal registration processes were widely available.
Core factors affecting email deliverability
While microdata might be the immediate trigger, it's rarely the sole reason an email gets flagged as dangerous. Gmail's filtering system is complex, relying on hundreds of signals to determine an email's legitimacy. These signals include your sender reputation, the authentication of your domain, and the overall content of your email. If any of these core elements are weak, the presence of microdata can simply amplify the suspiciousness.
For example, if your sender reputation is low, perhaps due to a history of complaints, sending to old lists, or being listed on an email blocklist (or blacklist), Gmail is already on high alert. Adding structured data, which is often used by legitimate brands, might be seen as an attempt to spoof or phish, leading to a dangerous message warning. Similarly, if your email authentication records like SPF, DKIM, or DMARC are misconfigured or failing, it weakens your sender's credibility, making any advanced features like microdata appear more suspicious.
Factor
Impact on Deliverability
Sender reputation
A low or questionable reputation is a primary flag for Google. Microdata can heighten suspicion if reputation is already poor.
Email authentication
Failing Gmail authentication checks (SPF, DKIM, DMARC) immediately raises red flags, making emails seem like phishing attempts.
Content and links
Suspicious keywords, unusual formatting, or links to untrustworthy domains can trigger warnings. Even legitimate links can be flagged if associated with poor reputation.
Absence of registration for markup
For some markup types and consistent display, Google requires prior registration or whitelisting of the sender.
Beyond technical validation: sender reputation is key
The primary way Gmail (and other mailboxes like Yahoo) determines trust is through your sender reputation and the robustness of your email authentication protocols. If your domain or IP address has a low reputation, it signals to mailboxes that your emails might be spam or even malicious. This is why you might see warnings like 'This message seems dangerous' warning appear.
To safeguard your sender reputation, regularly monitor your email deliverability and any blacklists (or blocklists) your domain or IP might appear on. Consistent authentication with SPF, DKIM, and DMARC is non-negotiable for establishing trust. Implementing a strict DMARC policy can significantly boost your credibility by protecting your domain from unauthorized use and clearly signaling to receivers that your emails are authentic. It's also important to improve domain reputation using Google Postmaster Tools for Gmail specifically.
Best practices for a healthy sender reputation
Authenticate: Ensure your SPF, DKIM, and DMARC records are correctly set up and pass alignment checks. This proves your email's legitimacy.
Monitor: Keep a close eye on your sender reputation using tools like Google Postmaster Tools and check if your domain or IP is on any blocklists.
Content quality: Avoid spammy keywords, excessive links, or misleading subject lines. Ensure your content is relevant and valuable to recipients.
List hygiene: Regularly clean your email lists to remove inactive or invalid addresses, reducing bounces and spam trap hits.
Strategic troubleshooting and solutions
If you suspect your microdata markup is indeed causing Gmail to flag your emails as dangerous, a systematic troubleshooting approach is essential. Start by isolating the markup. I've found success in performing A/B tests: send one version of your email with the markup and another identical version without it.
Observe the delivery results closely. If removing the markup resolves the phishing warnings or spam placement, then the markup is indeed a contributing factor. At this point, revisit Gmail's developer documentation on structured data to ensure you meet all requirements, including any sender registration processes. If you can't register or meet the criteria, it may be necessary to remove the markup for Gmail recipients.
With microdata markup
Emails consistently land in spam or are flagged as dangerous by Gmail.
Despite valid content and authentication, suspicion remains due to markup. The added structured data is not rendering as intended, and it triggers a negative response.
Without microdata markup
Emails are delivered to the inbox without Gmail's dangerous warning.
The core email content is delivered successfully, confirming the markup was the specific element causing the deliverability issue.
Remember, even if the markup is the proximate cause, the underlying issues of sender reputation and authentication should always be addressed. Ignoring these foundational elements will likely lead to other deliverability problems down the line, whether or not you use microdata. A healthy email program is built on strong authentication, good sending practices, and consistent monitoring.
Moving forward with email markup
Navigating email deliverability can be complex, and unexpected issues like microdata triggering Gmail's dangerous warnings are a testament to that. While structured data offers immense potential for enhancing email experiences, its implementation must be considered within the broader context of your sender reputation and email authentication.
My advice is to always prioritize the core principles of good sending: strong authentication, a clean sending list, relevant content, and diligent monitoring. If you choose to use microdata, ensure you meet Google's requirements fully, including any necessary sender registration. This holistic approach will not only prevent security warnings but also ensure your emails consistently reach the inbox, where they can be most effective.
Remember, deliverability is an ongoing process. Regular testing and adaptation to mailbox provider rules are key to maintaining a healthy sending program and avoiding unexpected flags, regardless of the advanced features you implement.
Views from the trenches
Best practices
Ensure full compliance with email authentication standards (SPF, DKIM, DMARC) for all sending domains.
Regularly monitor your domain and IP reputation using tools like Google Postmaster Tools.
Segment email campaigns to test markup effects on smaller, engaged audiences first.
Maintain a clean and validated email list to minimize bounces and spam complaints.
Common pitfalls
Applying microdata without prior sender registration or whitelisting with Gmail.
Ignoring low sender reputation issues, assuming markup will bypass them.
Using suspicious or untrustworthy links within email content, including those in markup.
Failing to conduct systematic tests (A/B testing) with and without markup.
Expert tips
If microdata causes issues, try JSON-LD. It’s Google’s preferred structured data format.
When troubleshooting, use Gmail's native spam reporting to see exact reasons for flagging.
Always validate your microdata code with Google’s Structured Data Testing Tool before sending.
For transactional emails, simpler markup often performs better than complex, interactive elements.
Marketer view
Marketer from Email Geeks says they found that adding microdata markup was directly causing their emails to be marked as dangerous by Gmail, and removing it resolved the issue.
2021-07-07 - Email Geeks
Expert view
Expert from Email Geeks says that while markup issues are unlikely to be the primary cause, if a sender is certain, it's worth checking if the mail without markup has the same URL and if prior registration was done.
Gmail. It's a puzzling situation many email senders face, and it often leads to a deep dive into what might be going wrong with otherwise well-intentioned emails.
Google requires senders to be registered and whitelisted. Without proper registration, even perfectly valid microdata can be seen as suspicious, leading to emails being marked as dangerous. This is often the case for senders who implemented markup years ago before formal registration processes were widely available.
Yahoo) determines trust is through your sender reputation and the robustness of your email authentication protocols. If your domain or IP address has a low reputation, it signals to mailboxes that your emails might be spam or even malicious. This is why you might see warnings like 'This message seems dangerous' warning appear.
Google Postmaster Tools and check if your domain or IP is on any blocklists.
Gmail's developer documentation on structured data to ensure you meet all requirements, including any sender registration processes. If you can't register or meet the criteria, it may be necessary to remove the markup for Gmail recipients.
