Summary
Email A/B testing can sometimes lead to unexpected deliverability issues, especially when the winning version is sent to the larger audience. Initially, smaller test segments may inbox without problems, but the subsequent full deployment of the winning variant experiences a significant drop in engagement metrics like open rates, while opt-out rates might also shift. This phenomenon often points to underlying factors related to send volume, content variations, or a shift in sender reputation thresholds at internet service providers (ISPs).
Key findings
- Volume and consistency: A dedicated IP requires consistent, high-volume sending. Inconsistent or lower daily/weekly volumes (e.g., below 100k emails daily or 500k weekly) can lead to volatility in deliverability, making large, infrequent sends more prone to issues. ISPs may flag sudden spikes in volume from an otherwise low-volume sender.
- Time-based factors: Even a few hours between the test send and the main deployment can impact deliverability. ISP filtering can change rapidly based on real-time feedback and overall network traffic. Sending large volumes at non-optimal times (e.g., Saturday) might exacerbate issues if the recipient base is less active then.
- Content variations: Even subtle changes in creative assets, such as imagery or external links, between test versions and the winning send can influence how spam filters perceive the email. If the winning version contains elements that trigger filters, it can lead to blocklisting or spam folder placement.
- Engagement metrics: A significant drop in open rates combined with a lower opt-out rate for the winning send suggests that emails are not reaching the inbox, as subscribers cannot opt-out of emails they do not see. This is a strong indicator of deliverability problems rather than content dissatisfaction.
- Lack of granular data: Some ESPs (Email Service Providers) do not provide detailed domain-level engagement metrics, making it harder to diagnose specific deliverability issues with particular ISPs like Gmail or Outlook. This lack of visibility can hinder effective troubleshooting.
Key considerations
- IP warming and consistency: Ensure your sending volume is consistent and sufficiently high for a dedicated IP. If your typical daily or weekly volumes are low or sporadic, consider if a shared IP might be more appropriate, or work to stabilize your sending volume to improve your IP warming process.
- Testing methodology: Evaluate your A/B testing strategy. If the test segments are small and the winner is sent much later to a very large list, the initial good performance might not reflect how the larger send will be received. Consider testing methods that minimize time variation or allow for full list deployment of tested versions, even if it requires manual list splitting.
- Content scrutiny: Rigorously check all content, especially external links, in your winning email variant. While a specific link might be benign, its presence or reputation could trigger filters. Review your email template for any elements that could negatively impact deliverability. For more on best practices, see Holistic Email Marketing's guide on A/B testing problems.
- Postmaster tools: Utilize Google Postmaster Tools and other ISP-specific feedback loops to monitor your domain and IP reputation. These tools can provide insights into spam complaints and filtering issues that your ESP might not expose.
What email marketers say
Email marketers often face a unique challenge with A/B testing where initial smaller test sends show good deliverability, but the subsequent large-scale deployment of the winning variant struggles. Marketers frequently attribute this to factors like send volume inconsistency on dedicated IPs, content elements in the winning email, and the timing difference between the test and the main send. They highlight the frustration of limited visibility into deliverability data from their ESPs, making root cause analysis difficult.
Key opinions
- Volume inconsistency: Many marketers suspect that inconsistent send volumes on a dedicated IP are a primary culprit. If typical sends are segmented and smaller, a sudden large send, even if broken up by a few hours, can be viewed differently by ISPs.
- Content impact: Marketers ponder whether specific content elements, particularly external links or image-heavy layouts, in the winning email version could trigger spam filters upon a larger deployment, even if they passed initial tests.
- Testing strategy flaws: Some marketers suggest that running A/B tests with significant time gaps or across different days introduces too many variables, making it hard to isolate the impact of the content being tested. They advocate for simpler testing methods or full list sends where possible.
- Engagement as a key metric: The drop in open rates paired with a corresponding drop in opt-out rates is seen as a strong indicator that emails are not reaching the inbox at all, as unsubscribes require the email to be seen. This highlights a clear deliverability issue, not just poor content performance.
Key considerations
- Dedicated IP suitability: Marketers should assess if their send volume truly warrants a dedicated IP. If volumes are frequently below 100k daily or 500k weekly, a shared IP might offer more stability in deliverability. For more insights on this, consider our guide on email testing best practices.
- Manual testing options: When ESP features are limiting, consider manually splitting your list and sending the winning version to the full audience in a single, consistent deployment to mimic non-A/B test scenarios.
- Seedlist testing: Utilize seedlist testing services to gain real-time inbox placement results before a large send, providing insights that ESP reporting might miss. More information on A/B testing can be found in Mailjet's complete guide to A/B testing.
- Advocacy for deliverability: Marketers often need to educate internal stakeholders about deliverability best practices, like consistent mailstream separation via subdomains, to prevent future issues.
Marketer view
Email marketer from Email Geeks describes encountering deliverability issues only when A/B tests are involved, specifically seeing a major drop in open rates and a corresponding dip in opt-out rates for the winning send. This indicates the emails are not even reaching the inbox for recipients to unsubscribe. They note that their IP reputation was only 'medium' on the day of the issue, despite typically being high, and they use a dedicated IP through Pardot. The marketer is particularly baffled because the test segments inbox fine, but the larger winning segment does not.
Marketer view
Email marketer from Email Geeks suggests that if the two test versions are too similar in performance, perhaps testing isn't necessary, or the test should be run as a full send to avoid confounding variables. They emphasize the challenge of isolating the impact of the test variable when also dealing with time-of-day differences. This highlights the importance of experimental design in A/B testing, where minimizing external factors helps ensure the observed differences are truly due to the variable being tested.
What the experts say
Email deliverability experts highlight that inconsistent sending volume on a dedicated IP is a common cause of volatility in inbox placement. While small, segmented test sends might perform well due to their limited volume, a subsequent large blast to the full audience, even a few hours later, can trigger ISP filters. Experts often recommend minimum send volumes for dedicated IPs and emphasize the importance of consistent sending patterns to build and maintain a strong sender reputation.
Key opinions
- Dedicated IP thresholds: Experts commonly advise that a dedicated IP requires a minimum daily send volume, often around 100,000 emails, or a weekly volume of 500,000, to maintain consistent reputation. Sending below this threshold can lead to volatility.
- Consistency over volume: Even more crucial than raw volume is consistency. Transactional senders with smaller daily volumes (e.g., a few thousand) can perform well on dedicated IPs if their sending is consistent, and engagement and complaint rates are excellent.
- ISP perception of tests: ISPs do not differentiate between 'test' sends and 'live' sends; they only see the total volume and the reputation signals associated with that volume. If combined test and main sends create a spike, it can negatively impact reputation.
- Time of day influence: The time of day can significantly affect deliverability. Filters can react differently based on overall network traffic and recipient engagement patterns at specific hours, even within the same day.
- Content variations and filtering: Even subtle content differences, particularly within images or linked content, can trigger content-based filters when a larger volume is sent, if the content is perceived as spammy or low-quality.
Key considerations
- Evaluate IP strategy: Regularly assess whether your dedicated IP is still the right choice given your current sending volume and frequency. If volume is insufficient or highly inconsistent, a shared IP might be more stable. This is especially true when considering a migration to a new ESP.
- Monitor real-time reputation: Beyond reported IP reputation, utilize tools like Google Postmaster Tools and other ISP-specific feedback mechanisms. These can provide a more nuanced view of how your mail is performing, especially for sudden drops in open rates.
- Content and domain reputation: Remember that your content and overall domain reputation play a significant role. Even with a good IP, problematic content can lead to blocklisting. Maintain high engagement and low complaint rates to protect your sender reputation. MoEngage offers a guide on how to dodge the spam folder.
- Educate internally: Often, the challenge is getting internal buy-in for deliverability best practices. Provide data and explain the implications of inconsistent sending or suboptimal IP usage to key stakeholders.
Expert view
Deliverability expert from Email Geeks explains that the ISP's filtering systems do not differentiate between a 'test' send and a 'live' send. They simply observe the volume of mail originating from an IP. If the combined test traffic and the full deployment of the winner constitute a significant spike relative to usual sending patterns, it can trigger throttling or filtering, regardless of the sender's internal testing methodology. This implies that senders must manage their total send volume, including all test portions, as part of their overall reputation management strategy.
Expert view
Deliverability expert from Email Geeks suggests that a send volume of 10-70k emails, 2-3 days a week, might not be ideal for a dedicated IP address. They imply that such inconsistent or lower volumes could lead to reputation volatility, especially for large, sporadic campaigns like monthly newsletters. This points to the need for senders to match their IP type (dedicated vs. shared) to their actual sending patterns and volume consistency.
What the documentation says
Official documentation and research on email deliverability consistently emphasize sender reputation, volume consistency, and user engagement as key factors for inbox placement. While A/B testing is a crucial optimization tool, its execution must align with best practices that account for ISP filtering mechanisms. Documentation often warns against large, inconsistent send volumes, rapid changes in content or sending patterns, and neglecting the impact of subscriber feedback.
Key findings
- Sender reputation is paramount: ISPs largely base deliverability decisions on the sender's reputation, which is built on consistent positive engagement (opens, clicks), low complaints, and minimal bounces. Any activity that deviates from established patterns, such as sudden volume spikes, can negatively impact this reputation.
- Volume and frequency: For dedicated IPs, maintaining a predictable and consistent sending volume is critical. Inconsistent sending or large jumps in volume (e.g., from test segments to a full send) can signal suspicious activity to filters, leading to throttling or blocklisting.
- Content and user feedback: ISP filters analyze email content for spam indicators and rely heavily on recipient feedback (e.g., spam complaints, manual inboxing/deleting). A/B test winners, if they contain elements that are unexpectedly flagged by a larger audience, can quickly accumulate negative signals.
- Monitoring is essential: Tools like Google Postmaster Tools provide critical data on IP and domain reputation, spam rates, and delivery errors, offering a crucial layer of insight beyond basic ESP reporting. Monitoring these metrics is vital for diagnosing deliverability issues.
Key considerations
- A/B test execution: When A/B testing, ensure that test segments are representative of the full list and that the winning variant's deployment integrates smoothly with existing sending patterns to avoid sudden volume spikes. For more on this, review Mailjet's advice on improving deliverability after opens.
- Reputation management: Prioritize consistent sender reputation by maintaining high engagement, minimizing spam complaints, and regularly cleaning your list of inactive subscribers and bounces. A strong reputation can buffer against minor content or volume fluctuations.
- Proactive monitoring: Implement robust monitoring for your IP and domain health. Sudden drops in inbox placement or spikes in spam complaints after a send should trigger immediate investigation into the campaign's content, list quality, and sending patterns. For example, understanding what happens when your domain is blocklisted can inform your strategy.
- Content quality: Ensure that the content of your winning variant adheres to best practices, avoiding spammy keywords, excessive imagery, or poor HTML. Even statistically significant content can perform poorly if it's not deliverability-friendly. Bloomreach provides an ultimate guide to mastering email deliverability.
Technical article
Email deliverability documentation from MoEngage indicates that two of the biggest factors affecting email deliverability are email campaign content and sender reputation. This means even if your technical setup is sound, poor content or a tarnished sender history can lead to emails landing in the spam folder. It implies that A/B test content changes could directly influence deliverability, particularly for the larger send.
Technical article
Email Deliverability Guide from Sender.net emphasizes that maintaining a healthy sender reputation is crucial for inbox placement. This reputation is built over time through consistent positive engagement, low complaint rates, and adherence to email best practices. Sudden changes in sending volume or content can trigger negative reputation signals. The guide reinforces that ISPs use reputation as a primary filter, explaining why a winning A/B test send, if it deviates from established patterns, could face issues.
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