Why is Gmail flagging messages as suspicious due to low sender reputation?
Michael Ko
Co-founder & CEO, Suped
Published 28 May 2025
Updated 15 Aug 2025
7 min read
Receiving a message from Gmail stating that your email is "likely suspicious due to the very low reputation of the sending domain" can be frustrating and alarming. It means your messages are not reaching the inbox, often landing in spam or being outright rejected. This issue directly impacts your communication and business operations.
Gmail, like other major mailbox providers, employs sophisticated algorithms to protect its users from unwanted emails. A core part of this protection is evaluating the sender's reputation. If your sender reputation is low, Gmail's filters become highly cautious, leading to messages being flagged or blocked. This isn't necessarily an error on their part, but rather an indication that something about your sending patterns or domain history is raising red flags.
Understanding why this happens is the first step toward resolving it. We will explore the factors that contribute to a low sender reputation and provide actionable strategies to improve your email deliverability to Google inboxes.
Understanding sender reputation
Sender reputation is essentially a trust score assigned to your sending domain and IP address by Internet Service Providers (ISPs) and mailbox providers, such as Gmail. This score determines how likely your emails are to reach the inbox versus being marked as spam or rejected entirely. A high reputation signals that you are a legitimate sender with consistent, desired email practices.
Several factors contribute to your sender reputation, including your email authentication records, spam complaint rates, bounce rates, and recipient engagement. Gmail's filtering systems constantly monitor these metrics to assess the trustworthiness of incoming mail. A sudden drop in reputation can occur due to changes in any of these factors, signalling potential issues to Gmail's algorithms.
It is important to regularly monitor your reputation metrics, especially within Google Postmaster Tools. This provides valuable insights into how Gmail views your sending practices. Even if your internal sender score seems good, Gmail might still flag messages if their system detects suspicious activity or a low domain reputation, as detailed in this discussion about Gmail flagging emails.
Common causes of low sender reputation
Several factors can negatively impact your sender reputation, leading to Gmail flagging your messages as suspicious. One of the most common causes is a high spam complaint rate. If recipients frequently mark your emails as spam, it quickly degrades your reputation. Similarly, a high bounce rate, indicating you are sending to invalid or non-existent email addresses, signals poor list hygiene and can hurt your standing.
Another critical factor is the lack of proper email authentication, including SPF, DKIM, and DMARC. These protocols verify that your emails are legitimately sent from your domain, preventing spoofing and phishing. Without them, mailbox providers cannot confirm your identity, making your emails appear suspicious by default. This is often why Gmail might say it cannot verify the sender.
Content that appears spammy, contains suspicious links, or includes unusual formatting can also trigger filters. Even legitimate emails can be flagged if they mimic characteristics of phishing attempts. Furthermore, if your sending IP address or domain is listed on a public blocklist (or blacklist), it significantly harms your reputation, potentially causing emails to be blocked or sent to spam, as explained by Mailgun's insights on Gmail blocking.
The impact of low reputation on Gmail delivery
Gmail's primary goal is to protect its users from spam, phishing, and malware. When your sender reputation is low, their systems interpret this as a potential threat. The 550-5.7.1 suspicious flag is a direct consequence of this assessment. It's not just about content anymore; it's about the trustworthiness of the sender. This can result in a range of delivery issues, from emails landing in the spam folder to being completely rejected by Gmail, similar to how emails are suddenly rejected by Gmail.
A low reputation can cause inconsistencies in delivery, even for emails that seem legitimate. For instance, a reputation bounce from Gmail can affect both marketing campaigns and crucial transactional emails. This means welcome emails, password resets, and order confirmations could all be delayed or undelivered, leading to a poor user experience and potential loss of business.
Furthermore, a persistent low reputation can lead to Gmail deferring mail with rate limits or placing your domain on internal blocklists, making it even harder to recover. It's a cyclical problem: low reputation causes delivery issues, which further degrades engagement and reputation, trapping your emails in a spam spiral.
The consequences of low sender reputation
Reduced inbox placement: Emails frequently land in the spam or junk folder, or are outright blocked.
Delivery errors: You may receive bounce-back messages with codes like 550-5.7.1 indicating reputation issues.
Brand damage: Recipients lose trust in your brand when emails are not received or appear suspicious.
Engagement decline: Lower open and click-through rates, impacting marketing and communication efforts.
Steps to improve your sender reputation
Improving your sender reputation with Gmail requires a multi-faceted approach, starting with robust email authentication. Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned with your sending practices. These protocols help Outlook and Yahoo (and of course, Gmail) verify your identity and prevent malicious actors from spoofing your domain, as is increasingly important with new sender requirements from Google and Yahoo.
Maintain a clean email list by regularly removing invalid or unengaged addresses. This reduces bounce rates and spam trap hits. Implement a double opt-in process for new subscribers to ensure genuine interest and prevent spam complaints. Segment your audience and send relevant content to improve engagement, as positive interactions (opens, clicks) boost your reputation, while negative ones (spam complaints, unsubscribes) can quickly degrade it. This is a crucial step in resolving a low Gmail domain reputation.
Gradually increase your sending volume, especially with new domains or IPs, a process known as IP or domain warming. Sending too many emails too quickly can appear suspicious. Regularly monitor your sender reputation using Google Postmaster Tools and other reputation checking services to identify issues early. Proactive monitoring allows you to address problems before they escalate into significant deliverability challenges.
Finally, ensure your email content is clean, relevant, and free of spam trigger words or suspicious formatting. Avoid overly promotional language in transactional emails. Offering an easy unsubscribe option in every email is also crucial. A simple, clear unsubscribe link can significantly reduce spam complaints, further bolstering your sender reputation and improving overall email deliverability.
Views from the trenches
Best practices
Implement strong email authentication (SPF, DKIM, DMARC) for your domain to build trust.
Maintain clean email lists by regularly removing inactive or invalid addresses.
Encourage positive recipient engagement through valuable and relevant content.
Monitor your sender reputation continuously using tools like Google Postmaster Tools.
Common pitfalls
Ignoring email authentication settings, making your emails appear unverified.
Sending emails to purchased or old lists, leading to high bounce and complaint rates.
Using generic or spammy subject lines and content that triggers filters.
Abruptly increasing sending volume from a new or low-reputation domain.
Expert tips
Focus on consistent sending practices to establish a predictable reputation with ISPs.
Segment your audience and tailor content to improve engagement metrics.
Address any blocklist (or blacklist) listings promptly to minimize deliverability impact.
Proactively test email deliverability before major campaigns.
Marketer view
Marketer from Email Geeks says they started seeing increased suspicious flags from Gmail around January 3rd, and they are trying to determine if it's new rules or client behavior.
2021-01-08 - Email Geeks
Marketer view
Marketer from Email Geeks says they can't confirm a general increase in these messages on their end.
2021-01-08 - Email Geeks
Rebuilding trust with Gmail
A low sender reputation leading to Gmail flagging messages as suspicious is a clear indicator that your email program needs attention. It is crucial to address the underlying causes, whether they are technical misconfigurations, poor list hygiene, or problematic content. By focusing on strong authentication, active list management, and positive recipient engagement, you can systematically improve your standing with Gmail.
The path to recovery might take time, as reputation is built and rebuilt over consistent, positive sending behavior. However, the effort is well worth it, ensuring your critical communications reach their intended recipients and maintain your brand's credibility. Continuous monitoring and adaptation to Gmail's evolving filtering criteria will be key to long-term deliverability success.
Gmail stating that your email is "likely suspicious due to the very low reputation of the sending domain" can be frustrating and alarming. It means your messages are not reaching the inbox, often landing in spam or being outright rejected. This issue directly impacts your communication and business operations.
Google inboxes.
Outlook and 
Yahoo (and of course, Gmail) verify your identity and prevent malicious actors from spoofing your domain, as is increasingly important with new sender requirements from Google and Yahoo.
