What causes delivery errors when no hard bounces are seen in the ESP?
Summary
What email marketers say15Marketer opinions
Marketer from Email Geeks suggests the errors are likely 421 deferrals caused by sending a lot of mail to the bulk folder, which triggers the error, and advises checking seed results for spam placement.
Email marketer from MailerQ shares that delivery problems can arise due to feedback loops (where recipients mark emails as spam) and spam traps. These do not always result in hard bounces but significantly impact deliverability and sender reputation.
Email marketer from Litmus states that delivery issues without hard bounces can stem from inbox placement problems. Emails might be going to the spam folder, or being filtered. Monitoring inbox placement is crucial to understand the issues.
Marketer from Email Geeks shares that the delivery errors are likely deferrals.
Marketer from Email Geeks explains that delivery errors closely match the percentage of deferral/transient errors in logs for a specific sender domain with Gmail contacts, noting the accuracy of GPT reporting.
Email marketer from Reddit user u/EmailNoob discusses that even without hard bounces, ongoing delivery issues could indicate IP or domain reputation problems. Email providers might be throttling or filtering emails based on sender reputation. Tools can be used to monitor these issues.
Marketer from Email Geeks advises checking deferral reasons since August for Gmail, suggesting a rise in "Mailbox full" errors could indicate the cause of delivery issues.
Email marketer from StackOverflow user explains that graylisting is a method some mail servers use to defend against spam. The mail server will temporarily reject any email from a sender it does not recognize. If the email is legitimate, the sending server will, according to specification, try again later and eventually the email will be accepted.
Email marketer from ActiveCampaign explains that soft bounces are different from hard bounces. Soft bounces are generally temporary issues like a full inbox or server issues while hard bounces are permanent, such as a non-existent email address.
Marketer from Email Geeks says the errors are temporary failures that eventually result in delivery on the final attempt.
Email marketer from Reddit user u/EmailExpert answers that "delivery errors" without hard bounces often indicate deferrals. This means the receiving server is temporarily refusing the email, but the sending server will retry. Common causes include high volume from a new IP or temporary server issues.
Email marketer from Mailjet shares that soft bounces are temporary delivery failures. These can occur because the recipient's mailbox is full, the server is down, or the message is too large. ESPs typically retry delivery for soft bounces.
Email marketer from EmailToolTester responds that delivery errors without immediate hard bounces could be caused by content filtering. The email's content might trigger spam filters, leading to delayed or blocked delivery, and not a hard bounce.
Email marketer from SendPulse responds that encountering delivery errors without hard bounces could be due to soft bounces, greylisting, or temporary server issues. They suggest checking sender reputation and ensuring proper authentication (SPF, DKIM, DMARC).
Email marketer from SparkPost shares that transient bounces (soft bounces) happen when a mailbox is full, the server is down temporarily, or the email is too big. The email provider continues to try and deliver the email for some time.
What the experts say4Expert opinions
Expert from Word to the Wise responds that high complaint rates can cause delivery issues, where emails might be filtered or sent to spam even without triggering hard bounces. Monitoring feedback loops is important.
Expert from Word to the Wise, Laura Atkins, explains that delivery issues even without hard bounces can indicate reputation damage. ISPs might be filtering or delaying emails due to sender reputation, even if the messages aren't explicitly bounced.
Expert from Email Geeks asks about the ESP's retry period and suggests the errors could be deferrals, with mail still queued for delivery or eventual failure.
Expert from Spam Resource shares that ISPs might implement throttling based on sender reputation. This means they could temporarily delay email delivery or limit the number of connections from a particular IP address, leading to delivery errors without hard bounces.
What the documentation says4Technical articles
Documentation from Google Postmaster Tools explains that deferred emails are temporarily rejected. Google will attempt to deliver them again over a period of time. If the email can't be delivered after multiple attempts, it will be dropped. Common reasons include the recipient's server being temporarily unavailable or the recipient's mailbox being full.
Documentation from Microsoft explains that a non-delivery report (NDR) can indicate various delivery problems, including temporary issues that don't result in a hard bounce. This includes issues with the recipient's server or network.
Documentation from RFC 5321 (Simple Mail Transfer Protocol) defines transient failures (4xx errors) as temporary conditions. The server is unable to accept mail at this time but *may* be able to accept mail later. The sender should retry delivery.
Documentation from Oracle describes SMTP reply codes in the 4xx range (e.g., 421, 450, 451, 452) as indicating transient negative completion status. This usually means the error is temporary, and the command might be successful if retried.