Why are emails suddenly rejected by Gmail?

Key findings

• Authentication issues: Gmail's updated sender requirements heavily emphasize proper email authentication, including SPF, DKIM, and DMARC. Failure in any of these can lead to immediate rejections.
• Sender reputation: A sudden drop in your domain or IP reputation with Gmail can trigger rejections. This can be influenced by spam complaints, bounces, or low engagement. Monitoring your Google Postmaster Tools is essential.
• Technical SMTP errors: Specific error codes, such as a 550 5.7.1 data stream error, indicate a technical rejection, often implying a misunderstanding of the SMTP protocol or an issue with the email's encoding.
• Transient issues: Sometimes, rejections can be temporary, resolving on their own if the issue was on Gmail's end or a momentary glitch in your sending system. However, this shouldn't deter investigation.
• Content and behavior: Spam-like content, unapproved attachment types, or missing subject lines can also trigger rejections, though these often lead to spam folder placement rather than outright rejection.

Key considerations

• Analyze bounce messages: The error message provided by Gmail is your first and most important clue. A 550-series code often indicates a permanent rejection due to policy violations or severe reputation issues.
• Review authentication records: Ensure your SPF, DKIM, and DMARC records are correctly set up and aligned. Even a small misconfiguration can cause issues.
• Monitor sender reputation: Regularly check your domain and IP reputation using Google Postmaster Tools. Look for sudden drops in reputation or spikes in spam complaints.
• Examine recent changes: Consider any recent changes to your sending infrastructure, email content, sending volume, or list hygiene practices. These are common triggers for deliverability issues.
• Check for blocklistings: Although less common for sudden widespread rejections by Gmail, it's always prudent to perform a blacklist check on your sending IP and domain, even though Google relies more on internal reputation signals.

Key opinions

• Transient nature: Some marketers find that sudden rejection issues resolve themselves quickly, leading them to believe it might be a temporary hiccup on Gmail's end rather than a persistent reputation problem.
• Reputation is key: A common first thought is that the sender's reputation with Gmail has taken a hit. Marketers are advised to use Google Postmaster Tools to gauge their standing.
• Bounce rate indicators: While some senders experience rejections, others might see stable or minimal bounce rates, suggesting the issue isn't universal and could be localized to specific sending practices or domains.
• No sender-side changes: Many marketers report no changes to their sending infrastructure or email content, which makes diagnosing the sudden rejection more challenging and points towards external factors.
• Shift to spam folder: Sometimes, emails aren't outright rejected but are moved to the spam folder, impacting engagement metrics like clicks and opens without generating a bounce.

Key considerations

• Detailed error messages: Always check the specific bounce error message. A 550 5.7.1 error, for instance, can indicate a permission or policy issue rather than a general reputation problem.
• Holistic view: The inbox is a constantly moving target. Marketers need to adapt and continuously monitor their deliverability, as what worked yesterday might not work today.
• Internal metric tracking: Beyond bounce rates, closely monitor internal metrics like open and click rates, especially for Gmail recipients, as these can signal issues like spam folder placement even without rejections.
• Sender behavior analysis: Even if no explicit changes were made, evaluate sending patterns. Are there subtle shifts in volume, content, or recipient engagement that could have triggered Gmail's filters? This is particularly relevant if transactional emails are impacted.

Key opinions

• SMTP protocol adherence: Experts suggest that a 550 5.7.1 error often means Gmail perceives the sender as not speaking SMTP, indicating a fundamental technical problem on the sender's side or a severe misinterpretation by the receiving server.
• Encoding issues: They recommend investigating email encoding, such as the use of high ASCII characters, non-standard encoding, or internationalized domains, as these can trigger technical rejections if not handled correctly.
• Technical vs. reputation: A sudden rejection with a specific SMTP error code is more likely a technical issue rather than a sudden, drastic drop in reputation, especially if it resolves quickly without intervention.
• ISP-side anomalies: If a sender has made no changes, experts often consider the possibility that the problem originated on Gmail's side, such as a temporary glitch or system update.
• Persistent monitoring: Even if an issue resolves, experts advise continuous monitoring and deep dives into logs to understand root causes, as deliverability is an ever-evolving challenge.

Key considerations

• Deep dive into logs: Beyond the basic bounce message, review full SMTP transaction logs for more granular details on why Gmail rejected the email. This can reveal specific policy violations or technical errors.
• SMTP standard compliance: Ensure your sending system fully complies with SMTP standards and RFCs. Non-standard implementations can cause unexpected rejections from strict receivers like Gmail.
• Content and header review: Beyond the body, inspect email headers for anomalies or non-standard practices that could trigger a technical rejection. Consider if new templates are introducing issues.
• Proactive reputation management: Even if the immediate issue is technical, maintaining a consistently high sender reputation is your best defense against future problems. Address any underlying issues that could lead to sudden drops in deliverability.

Key findings

• Authentication enforcement: Gmail's documentation explicitly states that emails must be authenticated with SPF and DKIM. DMARC alignment is also strongly recommended, with enforcement policies (p=quarantine or p=reject) being increasingly preferred for bulk senders.
• Spam complaint thresholds: Maintaining a very low spam complaint rate (ideally below 0.1%) is critical. Exceeding this threshold can lead to emails being sent to spam or even rejected outright.
• One-click unsubscribe: For bulk senders, providing a one-click unsubscribe mechanism in the email header is a mandatory requirement, failure to do so can result in deliverability penalties.
• Valid recipient addresses: High bounce rates, especially to non-existent addresses, negatively impact sender reputation and can lead to rejections. Regular list hygiene is therefore crucial.
• SMTP error codes: Documentation often clarifies the meaning of SMTP error codes like 550 5.7.1, which typically signifies a policy rejection due to suspected spam or authentication failure.

Key considerations

• Follow bulk sender guidelines: Adhere strictly to Google's bulk sender guidelines. These are updated periodically and non-compliance can lead to sudden delivery issues.
• Implement DMARC properly: Ensure your DMARC policy is correctly configured and that your SPF and DKIM authentication passes DMARC alignment.
• Monitor Postmaster Tools: Google Postmaster Tools provides specific insights into your domain's reputation, spam rate, feedback loop data, and authentication status with Gmail, making it an indispensable resource.
• Maintain consistent sending patterns: Sudden spikes in volume or changes in content that deviate from historical sending patterns can trigger spam filters, as outlined in deliverability best practices.