Email Blacklists (RBLs)
Summary
Email blacklists (RBLs - Real-time Blacklists) are databases of IP addresses and domains known for sending spam. Mail servers query RBLs before accepting email to filter malicious senders. Blacklisting causes immediate deliverability failure.
What Is Email Blacklists (RBLs)?
Email blacklists, also called blocklists or DNSBLs (DNS-based Blacklists), are maintained by anti-spam organizations. When a mail server receives an email, it queries multiple RBLs using DNS lookups to check if the sending IP or domain has a history of spam. Major RBLs include Spamhaus ZEN, SURBL, Barracuda, and SpamCop. Each maintains different criteria for listing and delisting.
Why Email Blacklists (RBLs) Matters for Email & DNS Security
Blacklist status determines whether your emails are delivered. Being listed on a major RBL like Spamhaus means instant spam folder placement or outright rejection. According to IntoDNS data, domains on Spamhaus blocklists experience 91% email rejection rates. Even shared hosting can cause blacklisting if a neighbor sends spam from the same IP range.
How Email Blacklists (RBLs) Works (Technical)
- 1.Mail server receives connection from sending IP
- 2.Performs reverse DNS lookup (PTR record)
- 3.Queries multiple RBLs using DNS (e.g., 203.0.113.1.zen.spamhaus.org)
- 4.If RBL returns DNS record: sender is blacklisted
- 5.Apply filtering based on RBL reputation (block, spam folder, or score increase)
- 6.Some RBLs check domain reputation, others check URLs in email body
Common Misconfigurations
❌ Using shared hosting IP
Consequence: Neighbor spam causes your IP to be blacklisted
How IntoDNS detects this: IntoDNS checks IP blacklist status across 15+ databases
❌ No blacklist monitoring
Consequence: Discover blacklisting only after deliverability fails
How IntoDNS detects this: IntoDNS provides real-time blacklist checking via API
❌ Ignoring spam trap hits
Consequence: Spam traps cause automatic blacklisting
How IntoDNS detects this: IntoDNS recommends list hygiene and double opt-in
How IntoDNS.ai Detects & Scores This
IntoDNS checks major RBLs including Spamhaus (ZEN, PBL, SBL, XBL), SURBL, URIBL, Barracuda, and SpamCop. Both IP-based and domain-based blacklists are queried. Results show which specific RBLs have listings and link to delisting procedures.
How To Fix Email Blacklists (RBLs) Issues
- 1.Identify which RBL has your IP/domain listed using IntoDNS
- 2.Visit RBL website and request delisting
- 3.Fix root cause (stop spam, secure compromised servers, add authentication)
- 4.For Spamhaus: https://check.spamhaus.org/
- 5.For shared hosting: consider dedicated IP or VPS
- 6.Implement SPF, DKIM, and DMARC to prevent future listings
- 7.Monitor blacklist status daily via IntoDNS API
References
Source: IntoDNS.ai – DNS & email security diagnostics
Last updated: 2026-01-14
Category: deliverability