Why did my email open rates drop after increasing sending volume on a shared IP?

Key findings

• Shared IP vulnerability: On a shared IP address, your sending reputation is inherently tied to the behavior of other users on that same IP. A sudden spike in your volume, or a change in a co-sender's activity, can lead to deliverability issues for everyone. This can result in your emails being directed to the spam folder, even if your domain reputation remains high in tools like Google Postmaster Tools. This is a key reason why email deliverability can suddenly drop.
• Volume spikes: A drastic increase in sending volume, such as jumping from 6k to 16k emails, without proper warming up can be seen as suspicious by mailbox providers. While Google might prioritize engagement over volume for established senders, other ISPs may be more sensitive to rapid scaling, particularly on shared infrastructure.
• List quality impact: Even a consistent 2-3% bounce rate, if it includes a significant portion of spam traps or invalid addresses, can become problematic at higher volumes. Mailbox providers might interpret this as poor list hygiene, leading to increased filtering. This can be a factor in what causes a sudden drop in open rates.
• New sender identity: Introducing a new 'from' address or sender name, even if on the same domain with verified SPF and DKIM, can sometimes trigger a reputation reset with some mailbox providers. They may need to build trust with this specific sending identity.
• Lack of granular data: Without the ability to segment open rates and bounce reasons by recipient domain, it's difficult to pinpoint which ISPs are causing the most significant drops, hindering targeted troubleshooting.

Key considerations

• IP warming strategy: While shared IPs sometimes require less aggressive warming than dedicated IPs, significant volume increases still necessitate a gradual ramp-up to build trust with ISPs. Ensure your warming plan accounts for these larger jumps. Learn more about gradually increasing email volume on EmailTooltester.com.
• List hygiene: Regularly clean your email lists to remove invalid addresses, unengaged subscribers, and potential spam traps. Even a seemingly low bounce rate can become an issue at scale if it indicates underlying list quality problems.
• ESP relationship: Maintain close communication with your ESP. They can provide valuable insights into the shared IP's reputation, access to tools like Microsoft SNDS data (if applicable), and detailed bounce codes that can help diagnose the problem.
• Engagement monitoring: Beyond open rates, closely monitor other engagement metrics such as clicks, replies, and unsubscribes. Low engagement can signal to ISPs that your emails are not desired, leading to poorer inbox placement.
• Content relevance: Ensure your content remains highly relevant and engaging for your audience, especially as volume increases. Irrelevant content can lead to lower engagement and higher spam complaints.

Key opinions

• Shared IP risks: Marketers frequently point to the inherent downside of shared IP pools: the risk of blacklisting due to the actions of other senders. Even if your own practices are clean, someone else's misbehavior can negatively impact your deliverability.
• Volume jump sensitivity: Many marketers suspect that a large, sudden increase in sending volume (e.g., from 6k to 16k emails) is the primary culprit behind open rate drops, regardless of prior warming. ISPs often prefer gradual scaling, particularly for new or recently migrated campaigns.
• Bounce rate interpretation: While a consistent low bounce rate might not have caused issues historically, marketers acknowledge that bounce reasons (like spam or invalid addresses) are crucial indicators. High proportions of these, even at low rates, can signal poor list quality to ISPs at higher volumes.
• Data limitations: A common frustration among marketers is the lack of granular data, such as open rates and bounces broken down by specific recipient domains (like Yahoo or Outlook). This data is essential for targeted troubleshooting.
• Sender name impact: Even with the same domain and proper authentication, using a new 'from' name for the first time can contribute to a drop in open rates as mailbox providers re-evaluate the sending identity.

Key considerations

• Proactive IP monitoring: Even on shared IPs, actively monitoring for blocklistings (blacklists) is crucial, as the actions of other senders can swiftly impact your deliverability. Tools that check for IP blocklists can be helpful. This is vital for maintaining a good sender reputation and avoiding issues like those seen with bad IP reputation.
• Leveraging ESP support: When deliverability drops on a shared IP, marketers emphasize the importance of requesting detailed bounce codes and IP performance data directly from their ESP. This information is often inaccessible otherwise and can reveal the root cause.
• Continuous list cleaning: Regularly pruning invalid and unengaged addresses from your lists is paramount. Even if a 3% bounce rate was acceptable at lower volumes, it can significantly degrade your sender reputation when scaled up. This ties into why emails go to spam after high bounce rates.
• A/B testing content: While not always the primary cause, marketers suggest A/B testing different content variations and checking for faulty links. Content quality and integrity still contribute to inbox placement and engagement.
• Holistic view: Deliverability is rarely a result of a single factor. Marketers understand it's a complex interplay of many variables, requiring a systematic approach to identify and address all potential issues.

Key opinions

• Engagement over volume: Experts often stress that mailbox providers, especially Google, prioritize recipient engagement over raw sending volume. If your audience is consistently interacting with your emails, a volume increase is less likely to cause issues, but lack of engagement can quickly lead to filtering.
• Shared IP limitations: While shared IPs can simplify sending for lower volumes, experts caution that they offer less control over reputation. Other senders' problematic behavior can easily lead to your IP being listed on a blocklist or receiving poor reputation scores, impacting all users on that IP. This is a common issue for shared IP campaigns.
• Bounce reason analysis: Bounces themselves aren't always the sole cause, but the specific reasons behind them (e.g., 'message bounced due to spam content') are critical diagnostics. Mailbox providers do monitor consistent patterns of invalid addresses, indicating poor list quality.
• Microsoft SNDS utility: Experts recommend using Microsoft's Smart Network Data Services (SNDS) to check for shifts in IP performance. While shared IPs may limit direct access for individual senders, ESPs should be able to provide this data to help diagnose issues.
• Complexity of deliverability: Deliverability issues are rarely attributed to a single cause. Experts consistently emphasize that it's a multi-faceted problem requiring a holistic troubleshooting approach, checking off many solved checkboxes.

Key considerations

• Accessing granular data: It's vital for senders to obtain detailed bounce messages and open rates per domain from their ESP to effectively troubleshoot. This data allows for precise identification of problematic networks or specific filtering reasons.
• Proactive ESP engagement: If you're on a shared IP, actively communicate with your ESP. They are your primary resource for understanding the shared IP's performance, reputation, and any issues affecting the pool. This applies even if you are curious about how Google Postmaster Tools IP reputation affects deliverability.
• List quality prioritization: Maintaining a clean and engaged list is a foundational deliverability practice. Consistent sending to inactive or invalid mailboxes, even at low rates, eventually signals poor list management to ISPs and triggers increased filtering.
• Content relevance and A/B testing: While not the single major factor, experts agree that content quality, including subject lines and any links, still contributes significantly to engagement and avoiding spam filters. Regular A/B testing can help optimize these elements.
• Understanding ISP policies: Different mailbox providers have varying thresholds and algorithms for identifying spam and assessing sender reputation. A strategy that works for one ISP might not for another. General best practices should be followed across the board, like those outlined on Mailgun's blog.

Key findings

• Gradual warming: Documentation frequently advises gradually increasing email volume, especially for new or inactive sending accounts, to build and maintain a positive sender reputation with mailbox providers. Sudden spikes can be perceived as suspicious.
• Shared IP risks: Many resources explicitly state that on shared IP addresses, the sending practices of other users can directly impact your deliverability. If others engage in spammy behavior, the entire IP's reputation (and thus your deliverability) can suffer, potentially leading to blacklisting or increased filtering.
• Engagement signals: Documentation highlights that low open rates, coupled with low engagement, signal to ISPs that your emails are not valued by recipients. This can result in your messages being routed directly to the spam folder, a primary reason for significant drops in open rates.
• Spam rate impact: Increased spam complaint rates over a short period are a critical negative signal. They can lead to severe consequences, including increased filtration, reduced delivery speed (throttling), and a higher bounce rate for future sends.
• Sender reputation factors: Key factors influencing whether emails end up in spam include the sending domain and IP address, the spam complaint rate, and the subscriber engagement rate. These elements collectively determine your sender's trustworthiness.

Key considerations

• Maintaining sender reputation: Documentation consistently stresses that good sender reputation is built over time through consistent, positive sending behavior. Any deviation, like sudden volume increases or poor list quality, can quickly erode this trust.
• List quality: Cleaning email lists and ensuring engagement is often cited as a fundamental best practice. High bounce rates or low engagement will negatively impact deliverability regardless of IP type.
• Content and authenticity: Beyond technical setup (like SPF, DKIM, DMARC), the relevance and quality of email content play a significant role. Engaging content reduces spam complaints and increases positive interactions, signaling good sender behavior.
• Monitoring deliverability: Regularly checking deliverability metrics, including open rates, bounce rates, and spam complaints, is essential. Early detection of issues allows for quicker remediation before severe reputation damage occurs. You can find email deliverability guides on this topic.
• Choosing the right IP strategy: For very large volumes or highly sensitive campaigns, documentation sometimes suggests considering dedicated IPs to gain more control over reputation, contrasting with shared IPs which are often suitable for lower volumes (e.g., under 100,000 emails per month). Find out more about this on Mailchimp's resources.