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How do email service providers classify and manage SMTP bounce codes for deliverability?

Summary

Email service providers (ESPs) play a critical role in interpreting and managing SMTP bounce codes, which are essential for maintaining good email deliverability. While SMTP codes provide a foundational layer of information regarding email delivery failures, their classification into 'soft' and 'hard' bounces, and subsequent management, often goes beyond a simple code-to-action mapping. ESPs employ sophisticated logic that considers not only the primary SMTP response but also extended codes, human-readable messages, and even the sending domain's historical reputation with specific mailbox providers.

What email marketers say

Email marketers often face challenges in understanding the nuances of SMTP bounce codes and their ESP's specific classifications. Their primary concern is how these classifications impact their sending reputation and campaign performance. Many marketers express a desire for more transparency and granularity from their ESPs regarding bounce handling, particularly when dealing with seemingly permanent errors that are initially categorized as soft bounces.

Marketer view

Email marketer from Email Geeks reports an SMTP 554 error for a disabled Yahoo mailbox, expressing confusion that their ESP categorized it as a soft bounce. They observed that the 'mailbox disabled' message indicated a permanent issue, which should ideally be a hard bounce, highlighting a disconnect between the bounce message content and ESP classification.

26 Nov 2019 - Email Geeks

Marketer view

Email marketer from an Email Marketing Forum explains that different ESPs have varied approaches to classifying SMTP bounce codes. They point out that a common code like 550 (mailbox not found) might be a hard bounce for one ESP but, in specific scenarios, could be handled with retries by another, depending on their internal bounce logic.

15 Oct 2023 - Email Marketing Forum

What the experts say

Deliverability experts consistently highlight that SMTP response codes provide only a piece of the puzzle in bounce management. They argue that ESPs cannot, and should not, rely solely on these codes for determining future sending behavior. Instead, a more holistic approach that incorporates pattern matching on human-readable messages, historical data, and specific ISP behaviors is necessary for accurate and effective bounce handling.

Expert view

Deliverability expert from Email Geeks clarifies that the term 'soft bounce' lacks a universal definition, meaning its interpretation is largely up to each ESP. This emphasizes the variability in how temporary delivery failures are categorized across different platforms.

26 Nov 2019 - Email Geeks

Expert view

Deliverability expert from Spam Resource states that successful bounce handling goes beyond simple SMTP codes, requiring a nuanced understanding of intent versus actual message delivery. They argue that deliverability metrics should reflect whether a message reached the intended recipient and was opened, not just if it was accepted by the receiving server.

10 Mar 2024 - Spam Resource

What the documentation says

Official documentation, primarily RFCs, defines the basic framework for SMTP response codes, categorizing them into temporary and permanent failures. However, these documents provide a high-level standard and do not dictate the granular, real-world classification logic that ESPs must implement for effective deliverability. The gap between standardized codes and practical application requires ESPs to develop sophisticated systems for interpreting and acting upon bounce information.

Technical article

Technical documentation from RFC 5321 outlines that SMTP reply codes are categorized into three main types: 2xx for success, 4xx for transient negative completion replies (temporary failure), and 5xx for permanent negative completion replies (permanent failure). This fundamental classification dictates the initial handling of delivery attempts.

01 Oct 2008 - RFC 5321

Technical article

Technical documentation from RFC 3463 specifies enhanced mail system status codes, which provide more specific details about delivery failures. For example, a 5.1.1 indicates a bad destination mailbox address, providing more context than a generic 550 SMTP code alone.

01 Jan 2003 - RFC 3463

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