Summary
Identifying domain blocks when emailing a small number of recipients necessitates a comprehensive approach. It involves scrutinizing SMTP bounce reasons and codes (particularly 5xx errors and 550 codes like 'Relay denied'), manually examining logs for hard bounces, and employing tools to monitor sender reputation. Testing with multiple accounts within the target domain, utilizing online blocklist checkers, and analyzing connection timeouts to specific domains can provide further clarity. Proactive measures include adhering to email best practices, such as proper authentication (SPF, DKIM, DMARC), maintaining a clean email list, warming up IPs gradually, and monitoring feedback loops.
Key findings
- SMTP Analysis: Analyzing SMTP bounce codes, specifically 5xx errors and 550 codes, provides key insights into potential blocks.
- Bounce Rate Formula: Calculating the ratio of attempts to bounces using the formula (# Attempts - # Bounces) / # Attempts, sorted ascending, helps identify completely blocked domains.
- Manual Log Review: Manually checking logs for hard bounces and recurring error messages is crucial for low-volume scenarios.
- Sender Reputation: Monitoring sender reputation scores, blocklist status, and connection timeouts to specific domains can reveal deliverability issues.
- Account Testing: Testing email delivery with multiple accounts within the domain can expose partial or account-specific blocks.
Key considerations
- Email Authentication: Proper email authentication (SPF, DKIM, DMARC) is vital to prevent spam classifications and blocks.
- List Hygiene: Maintaining a clean email list with valid addresses reduces bounce rates and prevents blocks.
- Gradual Warm-up: Warming up new IPs/domains gradually with increasing email volume is crucial for establishing a positive sender reputation.
- Feedback Loop Monitoring: Implementing and monitoring feedback loops (FBLs) helps identify spam complaints that can lead to blocks.
- Best Practices Adherence: Adhering to email best practices, as outlined by major email providers, is essential for preventing spam classification and ensuring deliverability.
What email marketers say
11 marketer opinions
Identifying a domain block when emailing only a few recipients requires a multi-faceted approach. It involves analyzing SMTP bounce codes for specific error messages, manually reviewing logs for patterns, and leveraging tools to monitor sender reputation. Testing with multiple accounts within the domain and using blocklist checkers can also provide insights. Maintaining good email practices, such as proper authentication, list hygiene, and gradual volume increase, are crucial preventative measures.
Key opinions
- SMTP Analysis: Analyzing SMTP bounce codes, especially 5xx errors, can reveal blocks. Specific codes like 550 with 'Relay denied' are strong indicators.
- Manual Log Review: Manually reviewing logs for hard bounces and specific error messages is crucial when dealing with low email volumes to identify patterns of consistent blocking.
- Blocklist Checkers: Using online blocklist checkers provides a general overview of IP/domain reputation, but it's not definitive for individual domain blocks.
- Sender Reputation Monitoring: Actively monitoring sender reputation using tools like Sender Score helps detect sudden drops that may indicate deliverability issues.
- Account Testing: Testing email delivery with multiple accounts within the domain can reveal partial or account-specific blocks.
Key considerations
- Email Authentication: Proper email authentication (SPF, DKIM, DMARC) is essential to prove email legitimacy and prevent blocks.
- List Hygiene: Maintaining a clean email list by removing invalid or inactive addresses reduces bounce rates and prevents blocks.
- Volume Increase: Gradually increasing email volume, especially for new IPs or domains, helps establish a good sender reputation and avoid triggering spam filters.
- Feedback Loops: Implementing and monitoring feedback loops (FBLs) is vital for identifying spam complaints, which can lead to blocks.
- Seed testing: Testing using a seedlist to identify inbox, spam or block results by provider.
Marketer view
Email marketer from EmailOnAcid suggests actively monitoring your sender reputation using tools like Sender Score. A sudden drop in your score could indicate deliverability issues, potentially due to blocks. Consistent monitoring allows for early detection and mitigation.
15 Apr 2025 - EmailOnAcid
Marketer view
Email marketer from GlockApps recommends testing using a seed list to see if your email hits the inbox, spam folder or is blocked. If you see a blocked result from a specific provider it can give an indication about overall deliverability.
27 Nov 2021 - GlockApps
What the experts say
4 expert opinions
Identifying a domain block when emailing only a few recipients involves analyzing SMTP bounce reasons and error codes. Specific codes like 550, coupled with messages such as 'Relay denied,' may indicate a block. A specific formula of (# Attempts minus # Bounces) / # Attempts can also help identify domains where all emails are bouncing. Additionally, a connection timing out to one specific domain is indicative of a manual block.
Key opinions
- SMTP Bounce Analysis: Analyzing SMTP bounce reasons is valuable for identifying blocked domains.
- SMTP Error Codes: Examining SMTP error codes, particularly 550 with 'Relay denied' or 'User unknown,' is vital.
- Bounce Ratio Formula: Using the formula (# Attempts - # Bounces) / # Attempts, sorted ascending, highlights domains where all or most emails bounce, indicating a potential block.
- Connection timeout: If a connection times out when emailing a specific domain that is indicative of a manual block/rejection.
Key considerations
- Bounce Interpretation: Carefully interpret bounce messages and codes to differentiate between temporary issues and permanent blocks.
- Data Analysis: Sort and filter bounce data to effectively apply the formula and identify potential blocks.
- Connection timeout: Distinguish connection timeouts to one domain vs general connection issues.
Expert view
Expert from Email Geeks shares a formula to identify potentially blocked domains when sending low email volumes: (# Attempts minus # Bounces) / # Attempts sorted ascending. Domains with equal attempts and bounces are highly likely to be blocked.
7 Feb 2023 - Email Geeks
Expert view
Expert from Email Geeks advises that SMTP bounce reasons are valuable for identifying blocked domains and explains that analyzing bounces is a worthwhile task.
26 Dec 2021 - Email Geeks
What the documentation says
3 technical articles
Identifying domain blocks requires careful analysis of SMTP server responses, especially 5xx errors, and Non-Delivery Reports (NDRs). Examining the text within these responses provides insights into the cause of the failure, potentially indicating a domain block or IP address being blocked. Adhering to email best practices outlined by providers like Google is crucial to avoid being marked as spam and consequently blocked.
Key findings
- SMTP Error Analysis: SMTP server responses in the 5xx range indicate permanent failures that may point to a domain block. The text portion of the response gives further details.
- NDR Review: Analyzing Non-Delivery Reports (NDRs) or bounce messages is crucial for identifying specific error codes or phrases indicating a blocked IP address.
- Best Practices: Adhering to email best practices, as defined by providers such as Google, is vital for preventing spam classifications and subsequent blocks.
Key considerations
- Error Code Interpretation: Understand the meaning behind different SMTP error codes and NDR messages to diagnose the cause of delivery failures accurately.
- Sender Reputation: Maintain a positive sender reputation by following email best practices to avoid triggering spam filters and being blocked.
- Compliance: Stay up-to-date with the latest email guidelines and policies to ensure continued deliverability and avoid blocks.
Technical article
Documentation from RFC Editor details that SMTP server responses, particularly the 5xx range, indicate permanent failures. Examining the text part of the SMTP response provides crucial insights into the reason for failure, which could point to a domain block. Analyzing these error messages is critical for diagnosing deliverability issues.
7 Jun 2022 - RFC Editor
Technical article
Documentation from Microsoft explains that analyzing Non-Delivery Reports (NDRs) or bounce messages is crucial. Look for specific error codes or phrases indicating that your email server's IP address has been blocked by the recipient's email server.
28 May 2025 - Microsoft
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