RBL (Realtime Block List)

What is an RBL (Realtime Block List)?

An RBL (Realtime Block List) is a dynamic database used by mail servers to identify and block IP addresses or domains known for sending spam, phishing, or other malicious email. When a mail server receives a message, it checks the sender’s IP address against one or more RBLs. If the IP appears on a list, the message can be rejected, quarantined, or flagged as spam before reaching the recipient’s inbox.

RBLs form part of the broader email reputation ecosystem. They help maintain a cleaner internet by discouraging abuse and improving email deliverability for legitimate senders. Administrators use RBL data to protect networks, reduce spam load, and detect compromised systems that might be sending unsolicited or fraudulent mail.

How RBLs Work

RBLs are DNS-based systems (often referred to as DNSBLs—DNS-based Block Lists). Each list contains IP addresses or domains that have been observed engaging in abusive or suspicious behavior. When an incoming email arrives, the receiving mail server performs a DNS query against the RBL provider to see if the sender’s IP or domain is listed.

Example of a lookup process:

If the query returns a positive response, the server knows that the IP address 192.0.0.2 is listed on the Spamhaus RBL. Depending on configuration, the server may reject the message, add a warning header, or deliver it to a spam folder.

Common RBL providers include:

• Spamhaus (ZEN, SBL, XBL, PBL)
• Barracuda Reputation Block List
• Proofpoint Dynamic Reputation
• SpamCop
• SORBS

Types of RBL Listings

RBLs track different categories of senders depending on their purpose and data sources. The most common types include:

• Spam sources: IPs or domains actively sending unsolicited bulk mail
• Open relays: Servers that allow unauthorized message relaying
• Compromised hosts: Infected machines or botnets used to send spam
• Policy listings: Networks with misconfigured or suspicious behavior
• Dynamic IP ranges: Consumer broadband addresses not intended for mail delivery

RBLs are updated frequently, often in near real time. Listings can be temporary or permanent depending on the severity and duration of the detected abuse. Once the source cleans up its behavior and requests delisting, many providers remove entries automatically after verification.

Impact on Email Deliverability

Being listed on an RBL can significantly affect a sender’s reputation and email performance. Messages from listed IPs may be delayed, filtered into spam folders, or rejected entirely. For businesses, this can lead to communication breakdowns, reduced engagement, and damaged brand reputation.

Best practices to avoid RBL listings include:

• Maintain valid SPF, DKIM, and DMARC configurations
• Monitor outbound mail for spikes or unusual patterns
• Use reputable email-sending infrastructure
• Regularly scan systems for malware or unauthorized use
• Honor unsubscribe requests promptly to prevent spam complaints
• Check reputation and listing status using tools such as MXToolbox or directly on RBL provider sites

Recovering from an RBL listing typically involves identifying and fixing the underlying issue (for example, compromised accounts or poor mailing practices) and submitting a delisting request through the provider’s process.

RBLs and DMARCeye

DMARCeye incorporates RBL intelligence into its reporting and reputation monitoring. By cross-referencing IP addresses found in DMARC aggregate reports with known RBL databases, the platform helps organizations detect whether their authorized or unauthorized senders are being flagged as spam sources.

DMARCeye alerts administrators to potential deliverability risks caused by blacklisted IPs or domains, enabling fast remediation before email performance suffers. By combining RBL data with authentication results, DMARCeye provides a unified view of domain reputation and helps ensure consistent, trusted message delivery across all mail channels.

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.