Why do emails go to spam, and how does high sending frequency affect deliverability?

Key findings

• Frequency sensitivity: Internet Service Providers (ISPs) and spam filters increasingly analyze sending frequency. Sudden spikes or consistently high volumes to unengaged recipients can trigger spam classifications, even for otherwise legitimate mail. Inconsistent sending volumes can be particularly detrimental, leading to confusion among spam filters.
• Engagement decay: High frequency without corresponding engagement (opens, clicks) can lead to a decline in sender reputation. Recipients are more likely to ignore, delete, or mark as spam emails they receive too often, negatively affecting overall deliverability. This also relates to how sender reputation is influenced.
• Unsubscribe rates: An excessive number of emails often leads to increased unsubscribe rates. A high unsubscribe rate signals to ISPs that your content is not valued by your audience, which can further impact your ability to reach the inbox.
• List hygiene importance: Regularly cleaning your email list to remove unengaged subscribers and invalid addresses is crucial. Sending to a clean, engaged list, even at a higher frequency, is less risky than sending to a large, unmaintained list at a lower frequency.

Key considerations

• Audience segmentation: Tailoring email frequency based on subscriber engagement levels and preferences can mitigate risks. Highly engaged segments can receive more frequent communications without negative repercussions, while less engaged segments should receive fewer, more targeted emails. Consider improving inbox placement through segmentation.
• Feedback loops: Monitoring spam complaints via feedback loops is essential. A rise in complaints, even if small, is a strong indicator that your frequency or content is unwelcome, and immediate adjustments are necessary.
• Testing and monitoring: Continuously test different sending frequencies and monitor key metrics like open rates, click-through rates, and complaint rates. This data will help you find the optimal frequency for your audience and maintain a healthy sender reputation.
• Content relevance: Regardless of frequency, ensuring your email content is highly relevant and valuable to your subscribers is paramount. Irrelevant content, even sent infrequently, can lead to disengagement and spam complaints.

Key opinions

• Frequency over-optimism: Many marketers believe that sending more emails will lead to better results, often influenced by internal pressure from marketing directors who may not understand the nuances of deliverability. This can result in a lack of consideration for potential deliverability issues.
• Subscriber fatigue: There's a strong consensus that very high email frequency leads to subscriber fatigue, causing recipients to unsubscribe or mark emails as spam, even from brands they initially liked. This is a common factor in why emails go to spam.
• Seasonal spikes: Some industries, like gifting, experience seasonal peaks where sending frequency naturally increases. Marketers in these sectors recognize the deliverability risks but often face business pressure to maintain high volume during critical periods.
• Engagement-based sending: Smart marketers implement strategies like segmentation to exclude unengaged subscribers from certain campaigns, even during high-volume periods, to protect their sender reputation and improve inbox placement. This is key for optimizing deliverability.

Key considerations

• Balancing business goals with deliverability: Marketers must advocate for strategies that balance revenue goals with long-term deliverability health. This includes educating leadership on the hidden costs of poor inbox placement.
• Effective segmentation: Utilizing advanced segmentation to target specific audiences with relevant content and appropriate frequency is critical for maintaining engagement and avoiding spam filters.
• Monitoring subscriber behavior: Pay close attention to unsubscribe rates, spam complaint rates, and open/click trends. These metrics are early warning signs of frequency issues.
• Preference centers: Implementing preference centers that allow subscribers to control their email frequency and content preferences can empower users and reduce unwanted communications.

Key opinions

• Reputation erosion: Experts agree that excessive sending frequency, particularly to disengaged segments, rapidly erodes sender reputation. This can lead to emails being consistently flagged as spam or blocked, irrespective of authentication (SPF, DKIM, DMARC). This impacts why emails go to spam.
• User experience is paramount: ISP filters are heavily influenced by recipient actions (opens, clicks, replies, deletes without opening, marking as spam). A high frequency that results in negative user signals will inevitably lead to poorer inbox placement.
• List quality over quantity: Sending to a smaller, highly engaged list at a moderate frequency will yield far better deliverability and ROI than blasting a massive, unsegmented list daily. This underpins the concept of best practices for email frequency and volume management.
• Authentication as a baseline: While proper authentication (SPF, DKIM, DMARC) is foundational, it does not guarantee inbox placement if other factors, like frequency and engagement, are neglected.

Key considerations

• Dynamic segmentation: Implement dynamic segmentation strategies that automatically adjust sending frequency based on real-time engagement data, suppressing less active users from daily sends.
• Gradual ramp-up (warming): When increasing sending volume or migrating to a new IP, follow a strict IP warming schedule to build sender reputation gradually.
• Opt-in management: Ensure robust double opt-in processes and transparent subscription management to set clear expectations with subscribers regarding email frequency.
• Monitor blocklists: Regularly check if your IP or domain is listed on any blocklists (also known as blacklists), as this often indicates a severe deliverability issue that can be exacerbated by high volume. Understanding how email blacklists work is critical.

Key findings

• Reputation scoring: ISPs assign a sender reputation score based on various factors, including spam complaints, bounce rates, and engagement. High sending frequency to a disengaged list can negatively impact this score, making it harder to reach the inbox. Google, for instance, provides data via Postmaster Tools.
• Thresholds and limits: Many ISPs have internal, often unpublished, thresholds for daily or hourly sending volumes from a given IP or domain. Exceeding these, especially for new or poorly reputed senders, can lead to throttling or blocks.
• Engagement feedback: Documentation often stresses that recipient engagement is a primary driver of inbox placement. If high frequency leads to low opens or high deletions, filters interpret this as unwanted mail, regardless of technical compliance.
• Spam trap hits: Aggressive sending to old or uncleaned lists increases the likelihood of hitting spam traps. This immediately and severely damages sender reputation, leading to hard blocks or blocklisting (also called blacklisting). Learn more about how spam traps work.

Key considerations

• Warm-up protocols: For new IPs or domains, adhering to strict warm-up schedules (gradually increasing volume) is crucial to build a positive sending reputation before high-frequency sending. This is a common topic in ESP migration and IP warmup documentation.
• Content relevance and targeting: Ensure that increased frequency is justified by highly relevant and segmented content. Generic content sent too often is a major red flag for filters.
• Subscription management: Provide clear and easy unsubscribe options, and ideally, frequency preference centers. This allows recipients to manage their subscriptions, reducing spam complaints and maintaining positive engagement signals.
• Feedback loop integration: Actively participate in ISP feedback loops to immediately remove subscribers who mark your emails as spam, preventing further reputation damage.