What do different SMTP bounce codes mean for email deliverability and blocks?
Summary
What email marketers say9Marketer opinions
Email marketer from EmailToolTester shares that 421 errors mean that the service isn't available, and 521 means that the domain doesn't accept mail
Email marketer from Stack Overflow user user12345 explains that implementing retry mechanisms for soft bounces (temporary failures) is important. They explain how this allows for delivery attempts during transient issues like temporary server outages.
Email marketer from Email Marketing Forum user EmailProUser shares the importance of analyzing server logs for SMTP bounce codes. They explain how this helps identify patterns and diagnose recurring deliverability issues.
Email marketer from SendGrid explains that SMTP response codes directly impact sender reputation. Frequent hard bounces can lead to blacklisting, whereas resolving soft bounces improves deliverability.
Email marketer from ActiveCampaign explains that high bounce rates negatively impact deliverability by signaling poor list hygiene to ISPs. It emphasizes maintaining a clean email list to avoid being marked as spam.
Email marketer from Reddit user u/EmailGuru shares that treating hard bounces as permanent errors and immediately removing those addresses from your list is crucial. They also explain how continually sending to these addresses negatively impacts your sender reputation.
Email marketer from Quora user Jane Doe shares the importance of immediately removing hard bounces from your email list to protect your sender reputation and maintain good deliverability.
Email marketer from Mailjet explains that 550 errors often indicate a permanent failure, such as the mailbox being unavailable or the recipient being unknown. They emphasize the importance of understanding these codes for troubleshooting deliverability issues.
Marketer from Email Geeks explains a PH01 is given if a URL in your email is on a blocklist and suspected for phishing.
What the experts say4Expert opinions
Expert from Email Geeks shares his experience with inconsistent block codes and highlights the reliability of Yahoo PH01 as an indicator of issues, enabling quick notification and content correction.
Expert from SpamResource explains that understanding email non-delivery reports (NDRs) and SMTP error codes is critical for diagnosing and resolving email delivery issues. They highlight that NDRs provide insights into why an email was not delivered, helping senders identify and address problems like invalid addresses or server issues.
Expert from Word to the Wise (Laura Atkins) explains the challenges in interpreting bounce messages due to their variability. She explains that some bounces are easily readable, while others require more effort to decipher, but understanding them is essential to improve deliverability.
Expert from Email Geeks explains that most ESP heuristics would work out "NOT A BLOCK" to "SPAM BLOCK" bounce.
What the documentation says6Technical articles
Documentation from IONOS details a list of SMTP codes and what they mean, for example it shares that "550 5.1.1 The recipient specified does not exist." means that the email address doesn't exist.
Documentation from ietf.org provides details about the SMTP protocol, including the structure and usage of reply codes. It establishes the fundamental standards for email transmission.
Documentation from SparkPost explains common SMTP bounce codes and how to fix them. Includes information about throttling issues (4xx codes) and permanent failures (5xx codes).
Documentation from ietf.org provides a detailed explanation of enhanced mail system status codes, which are used to provide more specific information about delivery failures than basic SMTP reply codes. These codes are structured as three numerical fields (class, subject, detail) to indicate the nature of the problem.
Documentation from Google Postmaster Tools distinguishes between hard and soft bounces. Hard bounces indicate permanent delivery failures (e.g., invalid email address), while soft bounces are temporary issues (e.g., mailbox full).
Documentation from Microsoft details Non-Delivery Reports (NDRs) in Exchange Online, which include SMTP error codes. It emphasizes that NDRs provide diagnostic information for resolving email delivery problems, including details on why a message was not delivered.