Why does my email A/B test inbox fine initially but then has deliverability issues after the winner is sent?
Matthew Whittaker
Co-founder & CTO, Suped
Published 6 Jun 2025
Updated 15 Aug 2025
10 min read
It can be incredibly frustrating to see your A/B test samples deliver perfectly, only for the winning email to encounter significant deliverability issues when sent to your full audience. I’ve heard this story many times, and it points to underlying factors that shift when you scale from a test segment to a large-volume deployment. It's a common trap where the initial low-volume sends don't trigger the same scrutiny from mailbox providers as a large-volume send does.
Let's look at a concrete example. Imagine sending two test campaigns, A and B, each to roughly 35,000 recipients. Both achieve a healthy open rate of around 28-29%. However, when the winning version (say, B) is sent to the remaining 70,000 subscribers just a few hours later, the open rate plummets to 13.37%. This dramatic drop, coupled with a lower opt-out rate on the main send (which can indicate emails aren't even reaching the inbox for recipients to unsubscribe), strongly suggests a deliverability problem. My goal is to break down why this happens and what can be done about it.
The core issue often lies in how mailbox providers assess sender reputation and manage inbound mail flow. While a small test send might slip through without much notice, a larger follow-up send can trigger thresholds that lead to throttling, spam folder placement, or outright blocking. Even if your Google Postmaster Tools (GPT) shows a 'High' domain reputation, this doesn't tell the full story across all providers or in real-time.
The role of sending volume and consistency
One of the most significant factors is the sudden change in sending volume within a short period. Mailbox providers, such as Gmail and Outlook, monitor your sending patterns closely. If you send 35,000 emails and then, a few hours later, send another 70,000 from the same IP, this can look like an anomaly, especially if your typical daily or weekly volume isn't consistently high. For a dedicated IP, consistency in volume is key to maintaining a healthy reputation and inbox placement. Inconsistent, bursty sending can be seen as suspicious, regardless of the quality of your list or content.
Mailbox providers do not distinguish between 'test' sends and 'main' sends in terms of how they impact your IP reputation. They simply see the volume of mail originating from your IP address. If the total volume sent within a specific timeframe (e.g., a day) suddenly spikes due to the main send of an A/B test winner, it can trigger their spam filters. This is especially true if you are on a dedicated IP but aren't sending a high, consistent volume on a daily basis. Many experts recommend a minimum of 100,000 emails daily, or around 500,000 per week, for a dedicated IP to maintain stable reputation.
Dedicated IP volume considerations
To effectively warm up and maintain a dedicated IP, a consistent and substantial volume of mail is crucial. Lower or inconsistent volumes can lead to volatility in your sender score, making your deliverability more unpredictable. Even transactional senders with lower daily volumes can succeed on dedicated IPs if their sending is highly consistent and their engagement and complaint rates are excellent.
If your sending volume for this specific monthly newsletter is significantly higher than your typical daily sending, or if your overall daily volume is often below the recommended threshold for a dedicated IP, this could explain the issues. The larger send might be seen as a sudden, unexplained burst of activity, which is a red flag for spam filters.
Content, engagement, and time of day
While the immediate culprit might seem to be volume, the content of your winning email and the timing of its deployment can also play a crucial role. Even if your A/B test variations seem similar, subtle differences in content, especially images or external links, can influence how spam filters react when sent at a larger scale. For instance, if the winning version contained an external link that wasn't present in the smaller test batches, or if it had a higher density of images, it could trigger different filtering rules at a higher volume.
Engagement metrics are a critical signal for mailbox providers. Although your test sends had decent open rates, the significant drop in the open rate for the winning send (from ~28% to 13.37%) is a clear indicator that a large portion of those emails didn't reach the inbox. Furthermore, the lower opt-out rate on the winning send also suggests a deliverability problem, as recipients can't unsubscribe from emails they never see in their inbox. This is why inbox placement is more important than just a high deliverability rate (which only means the email wasn't hard bounced).
The time of day can also influence deliverability, even for a few hours' difference. If your main send hits mail servers during a peak spam hour, or a time when your audience is less likely to engage, it can negatively impact your sender reputation for that specific send. Different verticals and audiences have optimal sending times, and missing that window can impact initial engagement and, consequently, deliverability.
Here's a breakdown of the observed performance:
Send Type
Sent
Open Rate
Click Rate
Opt-Out Rate
Test Send A
34,788
28.83%
0.57%
0.47%
Test Send B
34,789
28.65%
0.47%
0.53%
Winning Send
69,737
13.37%
0.34%
0.24%
This data clearly illustrates a sudden drop in engagement for the larger send. While the click rate was the winning criterion, the sharp decline in open rates for the main deployment highlights a fundamental issue with delivery to the inbox, rather than merely engagement with the content.
Sender reputation and ISP perception
Your sender reputation is continuously evaluated by mailbox providers. While Google Postmaster Tools provides a valuable overview, it's not the only factor. Other ISPs have their own internal reputation systems, and a sudden, large increase in volume (even if expected by you) can appear as an anomaly if it deviates from your established sending patterns. This can temporarily, or even significantly, damage your sender reputation.
Even if your domain reputation remains high, your IP reputation could be taking a hit during these large send windows. This is especially true if you're on a dedicated IP and your overall sending volume fluctuates widely or is generally lower than what's optimal for that setup. Mailbox providers expect a steady stream of good mail from dedicated IPs. Large, infrequent bursts can be misinterpreted.
IP vs. domain reputation
Your domain reputation (based on your domain name) is often more stable, reflecting your long-term sending practices. However, your IP reputation (the address from which your emails are sent) can be more volatile and react more quickly to short-term sending patterns and volume changes. A dedicated IP requires consistent, high-volume sending to maintain a stable, positive reputation.
Beyond volume, certain content elements or sending behaviors in the winning email could also subtly trigger filters when amplified across a larger audience. While you mentioned it was a creative asset style test, even the shift from typography to illustration could have an impact if one version uses more images, or if image-to-text ratio changed significantly. ISPs also monitor engagement metrics closely, and a drop in open rates on a large segment is a strong negative signal, even if the individual test segments performed well.
Strategies for A/B testing and deliverability
Given these challenges, refining your A/B testing strategy can help mitigate deliverability issues. First, reconsider the necessity of A/B testing in cases where initial test results are very close. If the difference in performance between your A and B variants is minimal, the benefit of choosing a 'winner' might not outweigh the potential deliverability risks of splitting and re-sending.
For monthly newsletters, an alternative approach could be to test different versions of the email to your full list on different weeks, if your ESP allows for this without being marked as redundant or spammy. This avoids the rapid volume spike. If manually splitting the list is the only option, consider sending the entire email campaign in one go, rather than splitting it into test and winner segments, especially for large lists or if you suspect volume inconsistency is an issue for your dedicated IP.
Monitoring goes beyond basic engagement metrics. Utilizing a seedlist testing service can provide valuable insights into actual inbox placement before a full send. This allows you to identify potential issues before they impact your entire campaign. You can also monitor your IP and domain blocklist status regularly.
Finally, ensure your email authentication, including SPF, DKIM, and DMARC, is correctly configured and aligned. While not directly related to A/B testing, any misconfiguration can exacerbate deliverability problems when mailbox providers are already scrutinizing your sending patterns due to volume or content changes. Regular checks of your email authentication setup are fundamental to robust deliverability.
Views from the trenches
Best practices
Maintain a consistent daily or weekly sending volume to build a strong IP reputation, especially if using a dedicated IP.
Segment your audience based on engagement, sending to your most active subscribers first to build positive signals for ISPs.
Monitor your deliverability metrics beyond open rates, including bounces, spam complaints, and read rates.
Use a seedlist testing service to check inbox placement for critical campaigns before the full send.
Ensure all email authentication protocols, such as SPF, DKIM, and DMARC, are correctly configured and pass validation checks.
Common pitfalls
Sending inconsistent, bursty volumes from a dedicated IP, leading to reputation volatility and throttling.
Assuming small test segment performance will perfectly reflect large-volume campaign deliverability.
Ignoring subtle content changes in A/B test winners that could trigger spam filters on a larger scale.
Not considering the specific optimal sending times for your audience and hitting peak spam hours.
Failing to separate mail streams by subdomain, which can negatively impact the reputation of your primary domain.
Expert tips
For dedicated IPs, aim for a minimum of 100,000 emails daily or 500,000 weekly for stable reputation.
If A/B test results are very close, consider sending the entire campaign in one go rather than a staggered test-winner deployment.
Investigate and implement a consistent email sending cadence, even for non-daily newsletters.
Recognize that content quality and engagement are paramount, even for repeated content, if the audience consistently engages.
Regularly review your email program for consistency in volume, content, and recipient engagement.
Marketer view
Marketer from Email Geeks says the sudden drop in open rates and opt-out rate on the winning send indicates a significant deliverability impact.
2020-08-31 - Email Geeks
Expert view
Expert from Email Geeks says mailbox providers do not differentiate between test traffic and regular traffic when assessing volume impact.
2020-08-31 - Email Geeks
Navigating your A/B test results
The pattern of initial A/B test success followed by deliverability issues on the winner send is often a clear signal that mailbox providers are reacting to changes in your sending behavior, primarily volume and consistency. While your domain reputation might appear stable, the bursty nature of A/B test deployments, especially on a dedicated IP with inconsistent overall volume, can negatively impact your IP reputation in the short term.
To mitigate these issues, prioritize consistent sending volumes, even if it means adjusting your A/B testing methodology to avoid large, sudden spikes. Regularly monitor your deliverability and inbox placement across various providers, and be prepared to adapt your sending strategy based on real-world performance data. A proactive approach to understanding and managing your sender reputation is key to ensuring your emails consistently reach the inbox.
Gmail and 
Outlook, monitor your sending patterns closely. If you send 35,000 emails and then, a few hours later, send another 70,000 from the same IP, this can look like an anomaly, especially if your typical daily or weekly volume isn't consistently high. For a dedicated IP, consistency in volume is key to maintaining a healthy reputation and inbox placement. Inconsistent, bursty sending can be seen as suspicious, regardless of the quality of your list or content.
