Email Blocklist Monitoring Guide

Step 1: Email Sent

Your mail server sends an email to a recipient

Step 2: DNS Lookup

Receiving server queries blocklist databases via DNS to check your sending IP/domain

Step 3: Blocklist Check

If your IP/domain is found in the blocklist, a positive response is returned

Step 4: Action Taken

Email is rejected, marked as spam, or flagged based on the receiving server's policy

Most widely used blocklist. Includes SBL (spam sources), XBL (exploited systems), and PBL (policy blocks). Critical to monitor.

Focuses on domains and URLs found in spam messages rather than IP addresses

Maintained by Barracuda Networks, widely used in enterprise environments

User-reported spam blocklist with automatic delisting after 24 hours of no reports

Spam and Open Relay Blocking System, tracks open proxies, relays, and spam sources

Three-level system (Level 1: single IPs, Level 2: ISP ranges, Level 3: countries)

High Spam Complaints

Recipients marking your emails as spam triggers automatic listing on many blocklists. Usually happens with purchased lists, poor list hygiene, or sending to unengaged recipients.

Compromised Accounts

Hacked email accounts or weak passwords allow spammers to send through your server, resulting in rapid blocklist additions.

Infected Servers

Malware or viruses on your mail server can turn it into a spam relay without your knowledge.

Spam Trap Hits

Sending to spam trap addresses (honeypots) indicates poor list quality or email harvesting practices.

Open Relays/Proxies

Misconfigured mail servers that allow anyone to send email through them are quickly blocklisted.

Poor Authentication

Missing or failing SPF, DKIM, and DMARC records make your domain vulnerable to spoofing and can trigger listings.

Emails bounce immediately with a 550 error code. Complete delivery failure to affected recipients.

Emails are delivered but automatically filtered to spam/junk, drastically reducing open rates.

Being listed signals poor sending practices to email providers, affecting overall sender reputation.

Even after delisting, it can take weeks for sender reputation to fully recover.

Manual DNS Lookups

You can check individual blocklists using DNS queries. For IP 192.0.2.1 on Spamhaus:

A response indicates you're listed. No response means you're clear.

Automated Multi-List Checking

Use blocklist checking tools to query 50+ lists simultaneously. Our Blocklist Checker provides:

• Real-time checks across major blocklists
• IP and domain reputation lookups
• Direct links to delisting procedures
• Historical tracking of listings

Monitor Email Bounce Messages

Bounce messages often indicate blocklist issues. Look for SMTP errors referencing specific blocklists (e.g., "550 5.7.1 Service unavailable; Client host [192.0.2.1] blocked using zen.spamhaus.org").

1. Identify the Blocklist

Check which specific blocklist(s) you're on using multi-list checking tools. Each blocklist has its own delisting procedure and requirements.

2. Investigate the Root Cause

Before requesting delisting, understand why you were listed:

• Review recent email sending patterns and volumes
• Check for compromised accounts or servers
• Analyze spam complaint rates
• Review list acquisition and management practices
• Verify server security and configuration

3. Fix the Underlying Issue

Take corrective action before requesting delisting:

• Close security vulnerabilities
• Change compromised passwords
• Remove malware or unauthorized scripts
• Clean your email list of invalid/unengaged addresses
• Implement proper authentication (SPF, DKIM, DMARC)
• Configure rate limiting and monitoring

4. Submit Delisting Request

Visit the blocklist's website and complete their delisting form. Common delisting URLs:

• Spamhaus: spamhaus.org/lookup
• SpamCop: spamcop.net/bl.shtml
• Barracuda: barracudacentral.org/rbl/removal
• SORBS: sorbs.net/delisting

5. Wait for Processing

Delisting can be immediate (SpamCop auto-expires after 24 hours) or take several days (Spamhaus reviews manually). Some blocklists require a waiting period before delisting to ensure the issue is resolved.

6. Verify Delisting

After delisting is confirmed, monitor closely for several weeks to ensure the problem doesn't recur. Test email delivery to major providers.

Implement Strong Authentication

Configure SPF, DKIM, and DMARC properly to prevent spoofing and demonstrate legitimacy. See our authentication guides.

Practice List Hygiene

Regularly remove bounces, unsubscribes, and inactive addresses. Use double opt-in for new subscribers. Never purchase email lists.

Monitor Engagement Metrics

Track open rates, click rates, and especially spam complaints. High complaint rates (above 0.1%) are a red flag.

Secure Your Infrastructure

Use strong passwords, enable two-factor authentication, keep software updated, and monitor for suspicious activity. Configure mail server security properly.

Implement Rate Limiting

Limit sending volumes per hour/day to prevent sudden spikes that trigger blocklist algorithms. Warm up new IPs gradually.

Use Dedicated IP Addresses

For high-volume senders, dedicated IPs prevent reputation damage from other users on shared infrastructure.

Provide Clear Unsubscribe Options

Make unsubscribing easy and honor requests immediately. Users who can't unsubscribe will mark as spam instead.

Daily Automated Checks

Set up automated daily monitoring of your sending IPs and domains across major blocklists. Use API-based monitoring for continuous surveillance.

Alert Configuration

Configure instant alerts when new listings are detected. Time is critical - the faster you respond, the less damage to your sender reputation.

Monitor Multiple IPs

If you use multiple sending IPs or domains, monitor all of them. A listing on a secondary IP can still affect overall deliverability.

Track Historical Trends

Maintain records of past listings and delistings to identify patterns and recurring issues. This helps improve long-term sending practices.

Comprehensive Coverage

Check 30-50 major blocklists, not just the top 5. Some ISPs use lesser-known lists that can still impact deliverability.

Integrate with Metrics Dashboard

Correlate blocklist status with delivery metrics, bounce rates, and complaint rates for a complete picture of email health.