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Summary

Email deliverability varies significantly across mailbox providers because each operates with a unique and proprietary set of filtering technologies and algorithms. These systems assess various factors, including sender reputation, content, and user engagement, with differing weights and thresholds. Discrepancies also arise from a provider's specific user demographics, their individual risk tolerance for blocking mail, and how they interpret authentication standards. What is considered acceptable mail by one provider may be aggressively filtered by another, making consistent inbox placement a continuous challenge for senders.

Key findings

  • Proprietary Algorithms: Each mailbox provider utilizes its own unique, often undisclosed, proprietary algorithms and filtering systems to assess sender reputation and email content.
  • Varying Criteria Weighting: Mailbox providers assign different weights and strictness levels to factors such as sender reputation, content analysis, recipient engagement (opens, clicks, complaints), and authentication protocols.
  • Diverse User Bases: The demographics and behaviors of a mailbox provider's customer base significantly influence their specific filtering criteria and what they classify as desirable or problematic email.
  • Differing Risk Tolerance: Mailbox providers have distinct levels of risk tolerance for blocking mail; some are far more aggressive in filtering policies than others, leading to varied inbox placement.
  • Volume's Effect: Higher volume domains, such as Google, can statistically 'hide' or absorb more problematic mail within their vast traffic, making it harder for senders to discern specific issues compared to smaller domains.
  • Higher Standards for Large Senders: Larger volume senders may be subject to more stringent deliverability standards at certain mailbox providers, an effect that appears to be increasing.
  • Authentication Enforcement: Even with crucial authentication protocols like SPF, DKIM, and DMARC, each mailbox provider interprets and enforces these standards differently, directly impacting deliverability.

Key considerations

  • Avoid Comparisons: Senders should avoid the flawed argument that email performing well at one mailbox provider (MBP) should automatically perform equally well at another, as each MBP has distinct and often higher standards for 'good' mail.
  • Monitor Across MBPs: It is crucial for senders to actively monitor their deliverability performance specifically for each major mailbox provider, rather than assuming uniform inbox placement across all.
  • Focus on Core Principles: Despite varying MBP standards, maintaining strong sender reputation, ensuring proper authentication (SPF, DKIM, DMARC), and focusing on high-quality, engaging content are foundational for improving overall deliverability.
  • Adapt Strategies: Recognize that deliverability issues with a specific mailbox provider often stem from their unique filtering approach. Senders may need to adapt their sending practices or content based on feedback from individual MBPs.

What email marketers say

11 marketer opinions

Email deliverability continues to differ among mailbox providers because each maintains distinct and often opaque anti-spam systems. These systems employ unique proprietary algorithms that evaluate incoming mail based on a varied blend of factors, including sender reputation, email content, and user engagement metrics. The specific weighting and strictness of these criteria, along with how each provider interprets and enforces authentication protocols, cause significant fluctuations in inbox placement for the same email campaigns across different providers.

Key opinions

  • Unique Filtering Logic: Every mailbox provider utilizes its own distinct, often undisclosed, filtering technologies and proprietary algorithms to assess incoming email, which can vary significantly in their strictness and design.
  • Weighted Factor Analysis: Mailbox providers assign differing importance and strictness to common deliverability factors such as sender reputation, email content analysis, and recipient engagement metrics (e.g., opens, clicks, complaints, bounces).
  • Authentication Interpretation: Even with crucial authentication protocols like SPF, DKIM, and DMARC, each provider applies its own interpretation and enforcement, directly influencing whether authenticated email lands in the inbox or spam.
  • Higher Scrutiny for Large Senders: Larger volume email senders may face more stringent deliverability standards at certain mailbox providers, such as Google, a trend that has reportedly increased over the past year.

Key considerations

  • Avoid One-Size-Fits-All Logic: Senders should recognize that what works for deliverability at one mailbox provider may not apply uniformly to others, making direct comparisons misleading and often invalid, especially when dealing with bad actors or dissimilar sending profiles.
  • Challenge of Explanation: It remains a persistent challenge for deliverability experts to fully explain the complex, provider-specific discrepancies in inbox placement to email marketers, despite a deeper understanding of the underlying causes.
  • Adaptation is Key: Successful deliverability necessitates a tailored approach, as issues with specific providers often demand adjustments to sending practices, content, or authentication configurations to meet their unique filtering criteria.

Marketer view

Email marketer from Email Geeks shares an anecdote where an affiliate spammer used the argument that their competitors could send mail just fine and demanded equal treatment, which allowed the mailbox provider to apply equal treatment to all, implicitly highlighting the invalidity of such comparisons when dealing with bad actors.

29 Oct 2024 - Email Geeks

Marketer view

Email marketer from Email Geeks admits to having used the argument 'we are only having issue with 'your system' and nowhere else' in the past, but now understands much better how such discrepancies can happen, acknowledging it's still difficult to explain this to marketers.

12 Jun 2024 - Email Geeks

What the experts say

4 expert opinions

The varied approaches to email filtering among mailbox providers explain the persistent differences in deliverability, as each sets its own unique standards. These discrepancies arise from their distinct technical infrastructures, proprietary algorithms, and varying risk tolerance for blocking mail. Furthermore, the unique demographic makeup and preferences of each provider's user base significantly influence what mail is considered acceptable, leading to a complex landscape where successful delivery at one provider does not guarantee the same at another.

Key opinions

  • Distinct Technical Standards: Each mailbox provider maintains unique technical standards and proprietary filtering algorithms, leading to variations in how they assess and treat incoming email.
  • Differing Risk Tolerance: Mailbox providers vary significantly in their tolerance for risk, with some adopting more aggressive blocking policies than others based on their internal metrics.
  • Diverse User Demographics: The unique demographics and preferences of a mailbox provider's customer base directly influence their filtering criteria, as what is acceptable to one user group may not be to another.
  • Volume's Impact on Detection: Higher volume mailbox providers can sometimes statistically obscure or 'hide' a greater amount of problematic mail due to the sheer volume of traffic, making deliverability issues harder to detect.

Key considerations

  • Provider-Specific Evaluation: Recognize that deliverability performance must be evaluated on a per-provider basis, as a positive outcome with one mailbox provider does not guarantee similar results with others.
  • Adaptive Sending Practices: Be prepared to adjust sending strategies, content, or technical configurations to align with the specific filtering behaviors and risk tolerances of individual mailbox providers.
  • Resist Uniformity Expectation: Understand that each mailbox provider operates with distinct internal thresholds and standards, making it unproductive to expect uniform deliverability across all platforms.

Expert view

Expert from Email Geeks explains that the argument 'but my mail to X is fine, tell Y that I'm not a bad sender' is a common point raised by senders, but mailbox providers have different standards, with some like Microsoft having a much higher threshold for 'good' mail and being happy to block mail that would be fine elsewhere. She also adds that higher volume domains like Google can statistically hide more 'ick' or problematic mail than smaller domains, as it's harder to 'lose in the noise' with fewer subscribers.

16 Mar 2022 - Email Geeks

Expert view

Expert from Email Geeks explains that Mailbox Providers (MBPs) have different demographics for customers, meaning what Gmail customers like vs. Comcast/Yahoo/GMX can be very different, making it entirely possible for the 'MBP X likes our mail but not MBP Y' scenario to occur.

17 Dec 2022 - Email Geeks

What the documentation says

6 technical articles

Email deliverability varies among mailbox providers because each maintains its own unique, proprietary systems for evaluating incoming messages. Platforms like Google, Microsoft, and others develop distinct algorithms and filtering logic, which assess factors such as sender reputation, email content, and user engagement. The specific weighting and thresholds applied to these criteria differ significantly across providers, resulting in disparate inbox placement for the same email.

Key findings

  • Individual Filtering Systems: Each mailbox provider, including Google, Microsoft, and others, uses a unique set of filtering technologies and proprietary algorithms to assess email.
  • Custom Algorithm Criteria: Mailbox providers implement their own internal criteria and data points, like sender reputation, content analysis, and user feedback, to determine email deliverability.
  • Varied Factor Weighting: Factors such as IP/domain reputation, email content, and recipient engagement are weighed differently by each provider's filtering systems, leading to varied outcomes.
  • Evolving Systems: Mailbox providers continuously update and refine their spam filters and algorithms, meaning deliverability criteria are not static.

Key considerations

  • Provider-Specific Analysis: Senders must analyze and understand deliverability performance on a per-mailbox provider basis, as success with one does not guarantee success with another.
  • Tailored Deliverability Strategies: Achieving optimal deliverability requires adapting sending practices and content to meet the specific and often unique filtering requirements of individual mailbox providers.
  • Ongoing Monitoring: Consistent monitoring of deliverability metrics across all major mailbox providers is crucial due to their constantly evolving and distinct filtering logic.

Technical article

Documentation from Google Postmaster Tools Help explains that Google, like other mailbox providers, utilizes its own proprietary reputation algorithms to assess sending domains and IPs. This means how an email is perceived and delivered to a Gmail inbox is based on Google's specific criteria, which differ from other providers' systems, leading to varied deliverability.

11 Oct 2024 - Google Postmaster Tools Help

Technical article

Documentation from Microsoft Docs explains that Microsoft's Exchange Online Protection (EOP) uses a distinct set of filtering technologies, including sender reputation, content analysis, and data from programs like Smart Network Data Services (SNDS). These unique, internal criteria mean that deliverability to Outlook.com or Microsoft 365 inboxes will differ from other providers who employ their own specific filtering logic and thresholds.

18 Jul 2022 - Microsoft Docs

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    Why does email deliverability differ across mailbox providers? - Basics - Email deliverability - Knowledge base - Suped