Summary
Emails can bounce after being opened multiple times due to a combination of factors including how 'opens' are measured, subsequent content and reputation analysis, changes in email address validity, and post-delivery filtering and blocking. The initial 'open' may simply be an asset download by a bot or security system. After an email is initially delivered and opened, email providers can perform a real-time analysis of URLs, content, and sender reputation. If this analysis flags malicious content, triggers spam filters, or detects inappropriate content, the email can be retroactively bounced. Email addresses may transition from valid to invalid over time or be marked as spam traps, leading to later bounces. Additionally, delayed greylisting, blocklisting (triggered by the sender's behavior or content), and a human blocking the sender post-viewing can all cause bounces. Feedback loops resulting from recipients marking emails as spam also contribute to this issue. Understanding MTA logs and sender engagement metrics are also key in resolving the issue.
Key findings
- Asset Download Opens: Measured 'opens' might be asset downloads by bots or security software.
- Post-Open Analysis: Email providers perform real-time content, URL, and sender reputation analysis after the initial open.
- Address Validity Changes: Email addresses can transition to invalid or spam traps after the initial send/open.
- Delayed Greylisting: Delayed greylisting can lead to bounces after the email has been initially opened.
- Dynamic Blocklisting: IP addresses can be added to blocklists after the initial send based on sender behavior/content.
- Inappropriate Content Filtering: Emails can be filtered and blocked after delivery if their content is deemed inappropriate.
- Human Blocking: Recipients can manually block senders after viewing the email, causing bounces.
- Temporary failures: Temporary failures (4xx errors) can occur post-DATA command because of content filtering or greylisting.
- Feedback Loops: Recipient spam reports significantly contribute to bounce rates.
Key considerations
- Monitor MTA Logs: Check detailed MTA logs to identify greylisting or filtering decisions.
- Maintain List Hygiene: Regularly clean email lists to remove invalid, inactive addresses and spam traps.
- Content Appropriateness: Ensure email content is appropriate and avoids triggering spam filters or blocklists.
- Sender Reputation Monitoring: Monitor IP address and domain reputation to avoid blocklisting.
- Recipient Actions: Be mindful of recipient actions such as manual blocking and spam reports.
- Engagement Metrics: Track engagement metrics (clicks, replies) to identify potential issues with content or targeting.
- Implement Email Authentication: Implement proper email authentication (SPF, DKIM, DMARC) to improve deliverability.
What email marketers say
11 marketer opinions
Emails can bounce after being opened multiple times due to several factors related to spam filtering, security measures, and changes in email address validity. Initial opens might trigger a secondary analysis of the email's content, links, or sender reputation. If this analysis identifies malicious content, spam indicators, or engagement anomalies (low clicks/replies), the email can be retroactively flagged and bounced. Email addresses can also transition from valid to invalid, triggering a bounce when a subsequent send is attempted after the initial opens. Greylisting and blocklisting can also cause delayed bounces. Asset downloads can also be confused with opens, and these downloads can trigger security protocols, resulting in a bounce.
Key opinions
- Asset Downloads vs. Opens: Measured 'opens' might actually be asset downloads by bots or security software, which can then trigger a bounce.
- Delayed Spam Filtering: Email providers can perform real-time analysis of URLs and content after the initial open, leading to a bounce if malicious content is found.
- Email Validation Changes: An email address can become invalid after the initial open, resulting in a bounce on subsequent interactions.
- Engagement Analysis: Spam filters analyze engagement metrics; low engagement after opens can flag an email as suspicious.
- Greylisting and Blocklisting: Delayed greylisting and post-delivery blocklisting can lead to bounces after the email has been initially opened.
Key considerations
- Monitor MTA Logs: Check detailed MTA logs to identify greylisting or filtering decisions.
- Implement Security Measures: Implement robust security filtering to prevent security software from triggering false positives.
- Maintain List Hygiene: Regularly clean email lists to remove invalid or inactive addresses.
- Monitor Engagement Metrics: Track engagement metrics and adjust sending practices to improve engagement rates.
- Review Content and Links: Regularly review email content and links to ensure they don't trigger spam filters or blocklists.
Marketer view
Email Marketer from Email Geeks suggests to check the detailed MTA logs: it's likely that first attempt was greylisted, and the filter reviewed the content and made a decision before the reattempt.
20 Oct 2024 - Email Geeks
Marketer view
Email marketer from SendPulse shares that Hard bounces indicate a permanent reason an email cannot be delivered and typically results from invalid email addresses. An address that was once valid may become invalid over time. While they don't explicitly explain multiple opens before a bounce, a hard bounce could be triggered after a system re-validates the email on subsequent interactions (opens).
19 Oct 2023 - SendPulse
What the experts say
5 expert opinions
Emails can bounce after being opened due to filtering systems, content evaluation, and blocklisting. Filtering systems can bounce messages even after viewing, and individuals can manually block senders, causing bounces. Remote hosts may reject emails after receiving content and checking links. Some systems evaluate entire messages before delivery or temporarily fail them for later evaluation, leading to redelivery attempts. Blocklisting after the initial send can also cause subsequent bounces. Moreover, content deemed inappropriate can lead to post-delivery filtering and blocking.
Key opinions
- Filtering System Rejection: Filtering systems can bounce messages even after they've been viewed.
- Manual Blocking: Recipients can manually block senders, triggering bounce responses.
- Remote Host Content Checks: Remote hosts may reject emails after receiving content and checking links.
- Delayed Content Evaluation: Some systems evaluate entire messages or use temporary failures for later assessment.
- Post-Send Blocklisting: IP addresses added to blocklists after the initial send can cause subsequent bounces.
- Content-Based Blocking: Inappropriate content can lead to post-delivery filtering and blocking.
Key considerations
- Monitor Filtering Systems: Understand and monitor the filtering systems in place.
- Content Appropriateness: Ensure email content is appropriate to avoid triggering filters.
- IP Reputation: Monitor IP address reputation to avoid blocklisting.
- Recipient Actions: Be mindful of recipient actions, such as manual blocking.
- Email Authentication: Implement proper email authentication (SPF, DKIM, DMARC) to improve deliverability.
Expert view
Expert from Email Geeks suggests that a remote host may receive the content, check the links, and then reject it after the DATA transfer.
11 Feb 2025 - Email Geeks
Expert view
Expert from Email Geeks explains some places see the whole message before they accept it for delivery. Some places do temp failures after DATA - but they can keep a copy of the message and evaluate it and then accept it when it's attempted for redelivery.
2 Jul 2024 - Email Geeks
What the documentation says
5 technical articles
Emails can bounce after being opened multiple times due to temporary failures, content and sender reputation issues, spam reports, and blocklisting. Temporary failures (4xx errors) can occur post-DATA command because of content filtering or greylisting. Systems like Exchange Online Protection can initially allow an email but later flag it based on content or sender reputation after it's opened, leading to a bounce. Feedback loops, where recipients mark emails as spam, also contribute to bounce rates. High spam rates detected by services like Gmail directly impact deliverability, causing bounces. Finally, being added to a blocklist, possibly triggered by behaviour after an email is opened, can also lead to bounces.
Key findings
- Temporary Failures Post-DATA: Temporary failures can occur after the DATA command due to content filtering or greylisting.
- Dynamic Spam Filtering: Email can be initially allowed but flagged as spam based on content or sender reputation after being opened.
- Feedback Loops: Recipient spam reports significantly contribute to bounce rates.
- Reputation-Based Blocking: High spam rates impact deliverability, leading to bounces.
- Blocklisting: Being listed on a blocklist, possibly due to post-open behavior, results in bounces.
Key considerations
- Content Analysis: Analyze email content to avoid triggering spam filters.
- Sender Reputation: Maintain a positive sender reputation to prevent dynamic spam flagging.
- Manage Feedback Loops: Monitor and address recipient spam complaints.
- Review Sending Practices: Review sending practices to reduce spam rates.
- Monitor Blocklists: Monitor IP and domain reputation on relevant blocklists.
Technical article
Documentation from RFC Editor details that temporary failures (4xx errors) can occur after the DATA command, indicating that the server has accepted the message content but is unable to deliver it immediately. This could happen due to various reasons, including content filtering or greylisting.
15 Aug 2024 - RFC Editor
Technical article
Documentation from AWS states that bounces occur for permanent or temporary reasons. Feedback loops, where recipients mark emails as spam, contribute to bounce rates. It's conceivable that a user initially opens an email but then marks it as spam, leading to subsequent bounces to the sender.
22 Jan 2024 - AWS Documentation
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