Blocklist

What is a Blocklist in email?

A blocklist (also known as a blacklist) is a database or list of IP addresses, domains, or email senders that have been identified as sources of spam, phishing, or other malicious activity. Mail servers use blocklists to automatically filter or reject messages from these sources, improving overall email security and reducing unwanted mail. Blocklists are maintained by security organizations, internet service providers (ISPs), or enterprises to protect users and maintain sender reputation standards.

When a message is received, the mail server checks the sender’s IP address or domain against one or more blocklists. If a match is found, the message may be marked as spam, delayed, or rejected entirely. This process helps prevent fraudulent or low-quality traffic from reaching user inboxes and supports email deliverability for legitimate senders who maintain clean reputations.

How Blocklists Work

Blocklists are distributed via DNS, allowing mail servers to query them in real time during message evaluation. Each blocklist has its own inclusion criteria, which might consider spam complaints, malware detections, open relays, or policy violations. A DNS-based query quickly returns whether the sender’s IP or domain is listed.

Example of a blocklist check:

If this query resolves to a specific return code, the sending IP is on the Spamhaus blocklist, a widely used anti-spam service. Other popular blocklists include Barracuda, Proofpoint, and SORBS.

Types of blocklists include:

• IP-based blocklists: Flag entire IP addresses or ranges associated with spam activity.
• Domain-based blocklists: Track specific sending domains or URLs used in malicious campaigns.
• Content-based blocklists: Focus on message content or URLs found in phishing and malware distribution.

Most modern mail systems consult several blocklists simultaneously to form a combined reputation score that influences message delivery decisions.

Managing and Avoiding Blocklist Inclusion

Being listed on a blocklist can severely impact a sender’s reputation and cause legitimate messages to be delayed or rejected. Regular monitoring and proactive management are essential for maintaining clean email operations.

Best practices include:

• Ensure proper SPF, DKIM, and DMARC alignment
• Use dedicated, reputable sending IPs and domains
• Avoid purchased mailing lists and maintain clean subscriber databases
• Monitor feedback loops and suppress inactive or invalid recipients
• Authenticate all sending systems and prevent open relay configurations
• Regularly check for listings using services like MXToolbox or Spamhaus

Quick remediation, such as stopping abusive traffic, fixing DNS issues, or submitting delisting requests, can help restore sender reputation once a listing occurs.

Blocklists and DMARCeye

DMARCeye enhances visibility into blocklist-related risks by correlating authentication results with IP reputation data. When analyzing DMARC aggregate reports, the platform identifies IPs that fail authentication or appear on major blocklists, alerting administrators to potential deliverability issues or compromised sources.

By monitoring both legitimate and unauthorized traffic, DMARCeye helps ensure that your authorized senders maintain a strong reputation while detecting any rogue systems or networks responsible for spam or phishing. This proactive insight supports compliance, brand protection, and consistently high inbox placement rates.

Sign up for a free trial of DMARCeye today and secure your email domain.

To learn more about DMARC and DMARC-related terms, explore the DMARCeye Glossary.