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Why do email bounce notifications differ and why might bounced emails be marked as read?

Summary

Email bounce notifications can vary significantly in their timing and appearance, and in some perplexing cases, a bounced email might even be reported as 'read'. These discrepancies stem primarily from the diverse configurations and operational philosophies of different mail servers and email service providers (ESPs). What one server treats as an immediate, permanent rejection (a hard bounce), another might initially accept and then later defer or bounce (a soft bounce), or even deliver before an internal process determines it should be bounced. The 'read' status for a bounced email often arises from the loading of a tracking pixel embedded in the email before the receiving server or client ultimately rejects the message or identifies it as undeliverable. This complex interplay of server behaviors and tracking mechanisms highlights the nuanced nature of email deliverability.

What email marketers say

Email marketers frequently encounter varied bounce notifications, which can be confusing given the expectation of consistent delivery feedback. Their experiences highlight the practical challenges of interpreting different bounce messages and understanding why an email might appear to be opened even after a bounce is reported. Many marketers recognize that these discrepancies often stem from the diverse ways different email services and recipient servers process incoming mail, rather than simple delivery failures. The primary concern for marketers is maintaining a clean email list and ensuring messages reach the inbox, which these inconsistent bounce behaviors complicate.

Marketer view

A marketer from Email Geeks notes that they received an immediate bounce notification for one email, which was their usual experience, but a different email sent later resulted in a delayed bounce after 10-15 minutes, prompting confusion about the differing behaviors.

31 Jan 2019 - Email Geeks

Marketer view

A marketer from Klaviyo Help Center explains that a soft bounce is always caused by a temporary reason, such as a recipient's inbox being full or their email server being momentarily down, contributing to why some bounces are delayed.

15 Sep 2023 - Klaviyo Help Center

What the experts say

Email deliverability experts highlight that the variability in bounce notifications and the paradoxical 'read' status for bounced emails are direct consequences of the complex and decentralized nature of the email ecosystem. There's no single standard for how mail servers process messages and generate non-delivery reports (NDRs). Factors like server load, filtering policies, reputation systems, and internal delivery queues all contribute to differing response times and the format of bounce messages. The 'read' phenomenon, in particular, is often attributed to the timing of tracking pixel loads versus the eventual determination of a delivery failure.

Expert view

An expert from Email Geeks explains that if different email domains are involved, there is no expectation that they will behave the same way in terms of bounce notifications.

03 Feb 2019 - Email Geeks

Expert view

An expert from Spamresource indicates that ISPs might initially accept an email into their system, even if it's later determined to be spam or malware, which can cause a deferred bounce notification instead of an immediate rejection.

05 Mar 2024 - Spamresource

What the documentation says

Official documentation from various email service providers and industry standards often outlines the technical reasons behind differing bounce notifications and delivery statuses. These sources typically define hard and soft bounces based on the permanence of the delivery failure and describe the SMTP codes associated with each. They also touch upon concepts like deferred delivery and internal processing queues, which explain why some bounces are not immediate. While specific documentation may not explicitly detail the 'read' status paradox, the underlying technical explanations for tracking pixel behavior and server responses can help clarify how such scenarios occur.

Technical article

Klaviyo Help Center documentation clarifies that a soft bounce is always caused by a temporary reason, such as when a recipient's inbox is full or their email server is momentarily down, explaining potential delays in bounce notifications.

15 Sep 2023 - Klaviyo Help Center

Technical article

Business News Daily explains that a hard bounce signifies an email cannot be delivered for permanent reasons, while a soft bounce indicates a temporary issue, thereby influencing the nature and timing of bounce back messages.

01 Oct 2023 - Business News Daily

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