Summary
Experts and documentation sources indicate that IPs can be listed as 'blocked' on SNDS without corresponding bounces due to factors beyond hard bounce criteria. These factors include Microsoft's internal filtering decisions, reputation assessments, spam complaints, spam traps, and content-based filtering. Mail might be filtered to junk, deleted silently, or face reputation-based blocking before bounces are generated. Additionally, SNDS data can be inaccurate or delayed, and issues like poor authentication, RBL listings, and sudden sending volume spikes can contribute to SNDS blocking.
Key findings
- Filtering Beyond Bounces: Microsoft's filters reject messages based on factors beyond hard bounce criteria, such as sender reputation, content, or spam complaints.
- SNDS Data Limitations: SNDS data can be delayed, inaccurate, or not real-time; don't rely on it as the sole source of truth.
- Reputation Matters: Internal reputation assessment of sending IPs greatly affects filtering decisions.
- Spam Traps Cause Blocks: Hitting spam traps can lead to SNDS blocking even if messages aren't bouncing.
- Authentication is Key: Authentication issues (SPF, DKIM, DMARC) can contribute to SNDS listings.
- RBL Listings Matter: Presence on RBLs (Realtime Blackhole Lists) can influence filtering.
- Traffic Spikes: Sudden sending volume spikes or irregular sending behavior can trigger filters.
- Inconsistent Data: Microsoft data (SNDS and bounces) may at times be inconsistent, requiring cross-referencing of data.
Key considerations
- Cross-Reference Data: Verify SNDS data with bounce logs, feedback loops, and other monitoring tools.
- Proactive Reputation Management: Actively manage and protect sender reputation.
- Spam Trap Avoidance: Implement strategies to avoid hitting spam traps.
- Authentication: Ensure proper email authentication (SPF, DKIM, DMARC) is in place.
- Volume Control: Control sending volume and avoid sudden spikes.
- Content Quality: Ensure content is high quality and avoids spam triggers.
- Investigate and Contact Support: If issues are encountered, consider reaching out to Microsoft support for investigation.
What email marketers say
14 marketer opinions
The consensus is that IPs can be listed as 'blocked' on Microsoft's SNDS even without corresponding bounce messages due to a variety of factors. SNDS data may be delayed, inaccurate, or reflect filtering decisions not tied to hard bounce criteria. Microsoft might filter emails to junk or delete them based on reputation, spam complaints, or content, resulting in no bounces. Issues with authentication (SPF, DKIM, DMARC), spam traps, RBL listings, sudden sending volume spikes, and sending spam-like emails can also lead to SNDS blocking. It is generally agreed that relying solely on SNDS is not enough and cross-referencing with other feedback loops and actual bounce logs is important.
Key opinions
- Data Inaccuracy: SNDS data can be delayed, inaccurate, or not real-time, particularly concerning spam complaints. Don't rely solely on SNDS for deliverability insights.
- Filtering vs. Bouncing: Microsoft may filter emails to junk or delete them based on factors like sender reputation, content, or spam complaints, without generating a hard bounce.
- Reputation Impact: Spam traps and a poor sending reputation can lead to SNDS blocking, even if emails aren't technically bouncing.
- Authentication Issues: Problems with email authentication (SPF, DKIM, DMARC) can contribute to SNDS blocking.
- External Blacklists: Presence on Realtime Blackhole Lists (RBLs) can influence Microsoft's filtering decisions and lead to SNDS blocking.
- Spam Complaints: Microsoft blocks IPs based on spam complaints received, this can happen even if bounces are not received.
Key considerations
- Cross-Reference Data: Don't rely solely on SNDS. Check your own bounce logs, feedback loops, and other deliverability monitoring tools.
- Monitor Reputation: Actively monitor and maintain your sending reputation to avoid being flagged by Microsoft's filters.
- Investigate Spam Traps: Investigate if spam traps are the cause of SNDS listings and work to remove them.
- Verify Authentication: Ensure proper email authentication (SPF, DKIM, DMARC) is in place to improve deliverability.
- Contact Support: Consider submitting a support ticket to Microsoft to investigate the cause of the SNDS blocking.
- Check Sending Practices: Sudden increases in sending volume or bad sending practices can affect your deliverability and trigger spam complaints leading to SNDS listings.
Marketer view
Marketer from Email Geeks explains he has a customer with IPs listed as blocked by SNDS for a long time, but had been delivering fine until recently. Also, mentions IPs listed as blocked that haven't sent email in years.
7 Jun 2022 - Email Geeks
Marketer view
Marketer from Email Geeks agrees that Microsoft's systems often contradict themselves and don't make sense.
20 Mar 2023 - Email Geeks
What the experts say
2 expert opinions
Experts agree that IPs can be listed as blocked on SNDS without corresponding bounces due to Microsoft's internal filtering decisions based on factors such as sender reputation, spam complaints, and spam traps. Mail is often filtered instead of bounced, and deliverability/reputation damage leads to blocks.
Key opinions
- Filtering Decisions: Microsoft's SNDS data reflects internal filtering decisions that extend beyond just hard bounce criteria.
- Reputation Impact: Sender reputation significantly influences whether emails are blocked, even if they don't result in a bounce.
- Spam Traps: Hitting spam traps is a common cause for IPs being listed on SNDS, separate from standard bounces.
Key considerations
- Proactive Reputation Management: Actively manage and protect your sender reputation to minimize the risk of being blocked.
- Monitor for Spam Traps: Implement strategies to avoid hitting spam traps and regularly monitor your lists for any indications of trap hits.
- Understand Filtering Criteria: Recognize that Microsoft's filtering criteria extend beyond simple bounce codes and encompass a broader assessment of sender behavior and content.
Expert view
Expert from Spam Resource shares that the IPs listed in SNDS are usually due to hitting spam traps, and this can be different from IPs that generate bounces because the deliverability and reputation damage causes the IPs to be blocked.
8 Feb 2025 - Spam Resource
Expert view
Expert from Word to the Wise explains that SNDS data reflects Microsoft's internal filtering decisions, which can be based on factors beyond hard bounce criteria, such as sender reputation or spam complaints. As such, mail may be filtered without a bounce being generated.
19 May 2023 - Word to the Wise
What the documentation says
5 technical articles
Technical documentation confirms that Microsoft's SNDS 'blocked' status indicates active filtering, not necessarily hard bounces. Filtering decisions are based on internal reputation assessments, spam complaints, content, and other factors beyond traditional bounce criteria. Email may be silently filtered, junked, or deleted. Overall, not all delivery failures result in hard bounces.
Key findings
- Active Filtering: SNDS 'blocked' status signifies Microsoft is actively filtering mail from that IP address.
- Reputation-Based Filtering: Microsoft's internal reputation assessment of sending IPs affects filtering decisions before hard bounces are issued.
- Beyond Bounce Criteria: Microsoft's filters can reject messages based on factors beyond hard bounce criteria, like content or sender reputation, resulting in silent filtering.
- Low Reputation = Spam: Low domain or IP reputation can cause emails to be marked as spam rather than bounced.
- Silent Failures: Not all delivery failures result in a hard bounce; some are handled silently via filtering rules.
Key considerations
- Prioritize Reputation: Actively manage and protect sender reputation to minimize filtering.
- Monitor Content: Ensure email content adheres to best practices to avoid triggering spam filters.
- Understand Filtering Rules: Be aware that receivers' filtering rules can result in silent failures without hard bounces.
Technical article
Documentation from Microsoft explains that SNDS data reflects their internal reputation assessment of sending IPs. This reputation can affect filtering decisions before hard bounces are issued, which might result from spam complaints or other factors that impact reputation.
13 Jan 2023 - Microsoft
Technical article
Documentation from Microsoft explains that a 'blocked' status in SNDS indicates Microsoft is actively filtering mail from that IP address. They may not always generate a hard bounce if the mail is simply being filtered to junk or deleted.
28 Dec 2023 - Microsoft
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