Is the emailtooltester.com email deliverability test data reliable?
Matthew Whittaker
Co-founder & CTO, Suped
Published 10 Jul 2025
Updated 17 Aug 2025
6 min read
Many email marketers and businesses frequently look to third-party tests to gauge the performance of various email service providers (ESPs) and understand their deliverability rates. A prominent example is the EmailTooltester deliverability test, which aims to provide insights into how well different platforms get emails into the inbox. However, it is essential to critically evaluate the methodology and limitations of such tests before drawing definitive conclusions about their reliability.
While these tests can offer a snapshot, the dynamic and complex nature of email deliverability makes it challenging to capture a consistently accurate picture with simplified testing approaches. Factors such as sender reputation, email content, recipient engagement, and mailbox provider algorithms are constantly evolving, influencing where an email ultimately lands.
Understanding what constitutes reliable deliverability data is key to making informed decisions for your email strategy. My goal here is to help you understand the nuances involved in email deliverability testing and to assess the real value of data from sources like EmailTooltester.com.
The limitations of testing methodology
One of the primary concerns with many third-party deliverability tests, including EmailTooltester's, is the methodology, particularly regarding sample size and email formats. Sending only a handful of plain text and HTML emails to a limited set of accounts introduces significant 'noise' into the results.
This small sample size can make the results highly variable and not statistically representative of real-world sending conditions. Actual email campaigns involve sending to thousands or millions of diverse recipients, across various domains, with different engagement patterns. A test that does not reflect this scale and diversity might show inconsistent or misleading performance indicators.
Furthermore, the type of content used in these tests, often generic email templates with third-party domains, can be problematic. If these templates or domains have a poor reputation due to widespread abuse or generic usage, it can skew the results, making an ESP look worse than it actually is, regardless of its underlying infrastructure.
Common testing pitfalls
Limited sample size: Testing with only a few emails or recipient addresses fails to account for the vast complexities of mailbox provider filtering.
Generic content: Using unoptimized or reused email templates can trigger spam filters, regardless of the sending platform.
Lack of warming: New IPs or domains need a proper warming period to build sender reputation, which these tests often overlook.
The disconnect with seed lists
Many deliverability tests, including EmailTooltester's, rely on seed lists to assess inbox placement. A seed list is a collection of email addresses at various mailbox providers, used to determine where a test email lands (inbox, spam, or missing).
While seed lists offer a convenient way to get quick deliverability insights, they are not always representative of actual email delivery to real user accounts. Mailbox providers like Gmail and Outlook personalize filtering decisions based on individual recipient behavior, engagement history, and sender reputation over time. This means that a test email landing in a seed account inbox does not guarantee the same for a regular recipient.
The dynamic nature of filtering algorithms means that deliverability can vary significantly from one send to the next, even for the same sender. A static seed list cannot fully capture these real-time variations, leading to potentially misleading deliverability rate assessments.
Seed list results
Static snapshot: Shows where an email lands on a predetermined list of addresses at a specific time.
Limited personalization: Does not account for individual recipient behavior or prior engagement.
Potential for bias: Some seed addresses might be more sensitive to spam triggers than real user inboxes.
Real-world deliverability
Dynamic and continuous: Influenced by ongoing sender behavior, recipient interaction, and evolving algorithms.
Highly personalized: Varies based on each recipient's unique mailbox settings and historical interactions.
Comprehensive factors: Depends on sender reputation, authentication, content, and list quality.
Achieving strong deliverability goes beyond just choosing a highly-rated ESP or relying on superficial tests. It hinges on fundamental email security and sending practices.
The foundation of good deliverability lies in robust email authentication protocols such as SPF, DKIM, and DMARC. These technologies verify that emails are legitimately sent from your domain, preventing spoofing and improving trust with mailbox providers.Properly configuring DMARC, for instance, gives you control over what happens to emails that fail authentication, protecting your brand's reputation.
Your sender reputation, a critical metric, is built over time based on your sending volume, complaint rates, bounce rates, and engagement. Mailbox providers assess this reputation to decide whether to place your emails in the inbox or the spam folder. Monitoring email blocklists (or blacklists) and maintaining low spam complaint rates are crucial for a healthy reputation.
Beyond technical configurations, content quality and audience engagement play significant roles. Sending relevant, valuable content to an engaged audience reduces the likelihood of spam complaints and boosts positive interactions, which signals to mailbox providers that you are a legitimate sender.
Content relevance: Provide valuable content that recipients expect and engage with.
List hygiene: Regularly clean your email lists to remove inactive or invalid addresses, reducing bounces.
Interpreting deliverability data effectively
Given the complexities, how should one interpret deliverability test data from any source? It is important to view such data as a single data point rather than a definitive statement of an ESP's universal performance.
A comprehensive understanding of your own email deliverability requires more than just a single test. It involves continuous monitoring of various metrics, analyzing DMARC reports, and using tools like Google Postmaster Tools and Microsoft SNDS for deeper insights into your sending reputation and inbox placement with major providers. These tools provide first-hand data on your domain's performance, which is far more reliable than generic tests.
Ultimately, the most accurate deliverability test for your specific needs will come from monitoring your own campaigns and understanding your audience's engagement. While third-party tests can be part of a broader research process, they should not be the sole basis for critical decisions. Regularly testing your emails and proactively identifying potential issues is essential.
Inferred from a small test email sample, rather than real user feedback.
Views from the trenches
Best practices
Focus on maintaining high sender reputation through consistent, valuable sending.
Implement strong email authentication: SPF, DKIM, and DMARC are non-negotiable.
Segment your audience and personalize content to boost engagement and reduce complaints.
Common pitfalls
Over-relying on generic third-party tests for a comprehensive deliverability assessment.
Ignoring your DMARC reports, which provide crucial insights into authentication failures.
Not warming up new IPs or domains before sending large volumes of email.
Expert tips
Use your own sending data and ISP feedback loops as the most reliable indicators of deliverability.
Understand that deliverability is a moving target and requires continuous monitoring and adaptation.
Prioritize email quality and recipient experience over chasing high scores on isolated tests.
Expert view
Expert from Email Geeks says the EmailTooltester article is SEO bait and not a well-designed experiment, thus no reliable conclusions can be drawn from it.
2023-12-06 - Email Geeks
Marketer view
Marketer from Email Geeks says they feel the test is superficial, noting that it likely doesn't include crucial steps like IP warming or setting up proper compliances and standards.
2023-12-06 - Email Geeks
The path to accurate deliverability insights
While third-party email deliverability tests like EmailTooltester.com's can offer some initial insights or comparative data, they often fall short of providing a truly reliable picture of an ESP's or your own domain's actual deliverability performance.
The inherent limitations of small sample sizes, reliance on seed lists that don't fully mirror real user behavior, and the omission of crucial factors like IP warming and dynamic sender reputation, make their results potentially misleading.
For genuine insights into your email deliverability, prioritize your own first-party data, monitor authentication and sender reputation rigorously, and understand that consistent best practices, rather than one-off tests, are the ultimate determinants of inbox success.
Gmail and 
Outlook personalize filtering decisions based on individual recipient behavior, engagement history, and sender reputation over time. This means that a test email landing in a seed account inbox does not guarantee the same for a regular recipient.
Microsoft SNDS for deeper insights into your sending reputation and inbox placement with major providers. These tools provide first-hand data on your domain's performance, which is far more reliable than generic tests.
