# Understanding IP Blacklists (RBL, Spamhaus, Barracuda) & Delisting

https://www.bihostnet.com/en/knowledgebase/article/ip-blacklist-rbl-spamhaus-barracuda-check-delist

## What are Real-time Blackhole Lists (RBLs)?

RBL providers like Spamhaus, Barracuda, and SpamCop aggregate IP addresses observed transmitting spam or malware. When an IP is blacklisted, recipient mail transfer agents reject connection attempts with 550 errors.

## Diagnosing Blacklist Status

Check your IP address via **mxtoolbox.com/blacklists.aspx** to query over 80 reputation providers concurrently.

## Common Causes of Listing

- Compromised email account credentials sending outbound spam runs.
- Injected PHP mailer scripts within outdated CMS installations.
- Unprotected contact forms abused by spam bots without reCAPTCHA.

## Delisting Procedure

1. **Eliminate the Root Cause:** Flush the outbound mail queue, reset compromised passwords, and patch vulnerabilities before applying.
2. Submit delist requests via official portals (check.spamhaus.org or barracudacentral.org).
3. Clean listings are typically cleared within 2 to 24 hours.