Summary
Delivery errors without hard bounces can arise from a variety of interconnected factors, primarily involving temporary deliverability issues and sender reputation impacts. Deferrals and soft bounces are common, often due to full inboxes, server unavailability, or greylisting, with ESPs typically retrying delivery. However, these temporary issues can also signal deeper problems related to sender reputation, where ISPs might throttle, filter, or delay emails based on perceived sender quality, high complaint rates, or negative feedback loops. Content filtering and inbox placement further complicate matters, with emails potentially being marked as spam or simply not reaching the inbox despite being delivered. Accurately diagnosing these errors requires careful analysis of ESP logs, monitoring sender reputation, reviewing email content, tracking inbox placement, and actively managing feedback loops and subscriber engagement.
Key findings
- Deferrals/Soft Bounces: Errors often involve deferrals and soft bounces due to temporary issues (full inboxes, server problems, greylisting).
- Reputation Damage: Delivery problems without bounces often stem from IP/domain reputation issues, leading to throttling or filtering.
- Content Filtering: Aggressive spam filters can prevent delivery due to content issues even without triggering hard bounces.
- Inbox Placement: Emails may deliver but land in spam; monitoring is essential to understanding delivery problems.
- Engagement Metrics: High complaint rates and spam traps affect deliverability, leading to filtering rather than bounces.
- Authentication: Issues are linked to the need for proper authentication protocols to be in place - SPF, DKIM, DMARC
Key considerations
- Check ESP Logs: Analyze logs for deferral reasons, error codes, and transient failure notifications.
- Monitor Reputation: Use tools to track IP/domain reputation and sender scores.
- Review Content: Analyze content for spam triggers and compliance with best practices.
- Monitor Inbox Placement: Use seed lists to monitor inbox vs. spam placement.
- Manage Feedback Loops: Implement feedback loops and complaint handling processes.
- Authentication: Ensure SPF, DKIM, and DMARC are properly set up and validated.
What email marketers say
15 marketer opinions
Delivery errors without hard bounces can arise from various temporary issues and underlying problems affecting email deliverability. Common causes include deferrals due to temporary server issues, full mailboxes, or greylisting. Other potential reasons are related to sender reputation, content filtering, spam placement, and engagement metrics such as complaint rates and feedback loops. Properly diagnosing these delivery errors requires analyzing ESP logs, monitoring sender reputation, checking content for spam triggers, and investigating inbox placement.
Key opinions
- Deferrals/Soft Bounces: Delivery errors are often caused by deferrals or soft bounces, which are temporary failures due to issues like full mailboxes or server downtime. ESPs typically retry delivery.
- Sender Reputation: Poor sender reputation can lead to throttling or filtering, even without hard bounces. ISPs may delay or block emails based on the sender's IP or domain reputation.
- Content Filtering: Email content that triggers spam filters can cause delivery errors without hard bounces. This includes specific keywords, links, or formatting.
- Inbox Placement: Emails may be delivered but end up in the spam folder. Monitoring inbox placement is critical for understanding deliverability issues.
- Engagement Metrics: High complaint rates and negative feedback loops can impact deliverability, even without bounces, leading to filtering or blocking by ISPs.
- Greylisting: Receiving servers might temporarily reject emails from unknown senders, causing delivery delays. If the sending server retries, the email should eventually be delivered.
Key considerations
- Check ESP Logs: Analyze ESP logs for deferral reasons, soft bounce codes, and other error messages to identify the root cause of delivery problems.
- Monitor Reputation: Use tools to monitor IP and domain reputation to identify potential issues and take corrective actions.
- Review Content: Review email content for potential spam triggers and ensure it adheres to best practices for email formatting and messaging.
- Monitor Inbox Placement: Use seed lists or other tools to monitor inbox placement and identify if emails are landing in the spam folder.
- Address Complaints: Implement feedback loops to monitor complaint rates and take steps to improve email quality and relevance to reduce complaints.
- Authentication: Ensure proper authentication is configured (SPF, DKIM, DMARC) to help improve deliverability and sender reputation.
Marketer view
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.
19 Apr 2022 - Email Geeks
Marketer view
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.
8 Dec 2022 - MailerQ
What the experts say
4 expert opinions
Delivery errors without hard bounces can stem from several issues. They may be due to deferrals, where emails are temporarily queued for retry. Additionally, sender reputation plays a crucial role; ISPs might filter or delay emails because of reputation damage or implement throttling. High complaint rates can also lead to emails being filtered into spam folders without triggering hard bounces. Monitoring retry periods, sender reputation, and feedback loops are important for troubleshooting these issues.
Key opinions
- Deferrals: Delivery errors could be deferrals where emails are queued for retry.
- Reputation Damage: Delivery issues without hard bounces may indicate reputation damage, leading to filtering or delays.
- Complaint Rates: High complaint rates can cause filtering into spam folders without hard bounces.
- Throttling: ISPs might implement throttling based on sender reputation, delaying or limiting connections.
Key considerations
- Check Retry Periods: Determine ESP's retry period to assess if errors are due to temporary deferrals.
- Monitor Sender Reputation: Regularly monitor sender reputation to identify and address potential damage.
- Monitor Feedback Loops: Implement and monitor feedback loops to understand and address high complaint rates.
- Assess Throttling: Investigate potential throttling by ISPs based on connection limitations or delays.
Expert view
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.
11 Jul 2021 - Word to the Wise
Expert view
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.
23 Jul 2023 - Word to the Wise
What the documentation says
4 technical articles
Delivery errors without hard bounces often stem from temporary or transient issues, as indicated by email protocol documentation. Deferred emails, as per Google Postmaster Tools, are temporarily rejected but retried. RFC 5321 defines these as transient failures (4xx errors), suggesting the server is temporarily unable to accept mail, and delivery should be retried. Microsoft's documentation on non-delivery reports (NDRs) acknowledges that temporary issues can cause delivery problems without leading to hard bounces. Similarly, Oracle documentation points to 4xx SMTP reply codes as signs of temporary negative completion status, recommending retries. Thus, transient server problems, recipient inaccessibility, or network issues are typical causes of these delivery errors.
Key findings
- Deferred Emails: Google Postmaster Tools identifies deferred emails as temporarily rejected but subject to retries.
- Transient Failures (4xx Errors): RFC 5321 defines transient failures using 4xx SMTP error codes, suggesting temporary server inability.
- Non-Delivery Reports (NDRs): Microsoft's documentation confirms NDRs can indicate temporary delivery problems not resulting in hard bounces.
- Temporary Completion Status: Oracle documentation indicates 4xx SMTP reply codes as transient negative completion status, warranting retries.
Key considerations
- Check Retry Configuration: Confirm that the email sending system is configured to automatically retry sending deferred or temporarily failed messages.
- Monitor Error Codes: Pay attention to SMTP error codes, especially 4xx errors, as they indicate the specific type of temporary issue encountered.
- Inspect NDR Details: Examine the details provided in NDRs for insights into the cause of temporary delivery failures.
- Address Server Issues: Address any underlying server or network issues contributing to temporary delivery problems.
Technical article
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.
9 Aug 2023 - Google Postmaster Tools Help
Technical article
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.
18 Dec 2024 - Microsoft Learn
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