Imagine that an IP address is like an apartment number in a huge building called the Internet. When one of the residents is constantly making noise, littering or violating the rules, the neighbors decide: “That’s enough, we won’t let this person in anymore.” The same thing happens with IP: if an address behaves suspiciously, it can be added to the IP blacklist.
An IP blacklist is a list of addresses that are denied access to certain sites, services, or networks. It works like a blacklist of phones in a smartphone: you have added a number, and there will be no more calls from it. How is the list formed? This can be an automatic system that notices suspicious activity (for example, too many requests per second), or an administrator who manually blocks a “noisy neighbor.”
Where such lists are most often found:
- websites and online services to keep bots and intruders out;
- Email systems to filter out spam;
- Corporate filters that restrict access to unwanted resources.
How IP Blacklists Work
In fact, the IP blacklist is just a database with “bad” addresses. When the device or server tries to connect to the site or send an email, the system checks: “Is this IP in our list?” If yes, access is prohibited.
The mechanism can be:
- Automatic. Specialized algorithms analyze behavior: too many logins per minute, sending hundreds of emails, suspicious requests. The system adds the IP to the list by itself.
- Manual. The administrator adds the address manually if he notices abuse.
Who maintains such lists?
- Internet service providers — they can block IP addresses by law or internal regulations.
- Site and server administrators set up their own lists.
- International databases (DNSBL, etc.) — publish lists of “bad” addresses used by mail and network services around the world.
In fact, it’s like a black mark: you get into the database, and then your access is limited in a variety of places.
Main Reasons Why an IP Gets Blacklisted
IP addresses are blacklisted for a reason. There is usually a specific reason behind this.
Main scenarios:
- Sending spam. If hundreds of identical emails leave an IP address, mail services are almost guaranteed to add it to the blacklist.
- Suspected hacking or bruteforce. When the system sees dozens of failed login attempts in a short time, it looks like an attack. Such an IP address is quickly blocked.
- Violation of the rules of the services. Websites and online platforms can add IP addresses to the list if a user systematically violates the rules, for example, by creating dozens of fake accounts.
- Massive use of a single proxy or VPN. When hundreds of people access the network via the same IP address, anti-fraud systems may decide that this is suspicious activity and block the entire address.
Thus, not only intruders get into the blacklist, but also ordinary users if they happen to be “neighbors” through a suspicious proxy or VPN.
Types of IP Blacklists
There are several types of IP blacklists, and each solves its own problems.
- DNSBL (Domain Name System Blacklist). One of the oldest and most common types. It works through the DNS system: the server checks the IP and compares it with a database of spammers or suspicious addresses.
- Email blacklists. Specialized lists for mail servers. If an IP address is included in such a list, emails from it will almost always go to the Spam folder.
- Firewall / server blacklists. They are configured at the server and firewall levels. They block IP addresses that are detected in attacks, DDoS, or other abuses.
- Local blacklists. Internal lists within companies, organizations, or even home routers. For example, an admin can deny access to the network to a specific IP inside the office.
| Type of IP Blacklist | Description | Common Use Case |
|---|---|---|
| DNSBL | Domain Name System Blacklist; checks IPs against spam/spamhaus-style lists | Email filtering, anti-spam protection |
| Email Blacklists | Specialized lists of IPs used for sending spam | Preventing unwanted emails, improving inbox security |
| Firewall Blacklists | Server- or firewall-level blocklists | Blocking malicious traffic, brute-force protection |
| Local Blacklists | Custom lists maintained by organizations or admins | Internal network control, office policies |
Thus, the IP blacklist can be global (available to many services) or local (limited to a single network).
Consequences of Being on an IP Blacklist
How does a user and a business feel when an IP is blacklisted?
- Restriction of access to websites and services. You may be greeted by captchas, block pages, 403/451, and sudden logouts. Advertising cabinets and markets often include additional checks or do not allow them at all.
- Emails go to Spam. Massive drops in the Open Rate/CTR, complaints of “letters not reaching”, refunds with codes 550/5.7.1. Even transactional letters (checks, OTP) may not be delivered.
- Application slowdowns. Increased delays due to additional filters, API throttling, frequent captchas — users think that the “service is slowing down”, although the problem is the IP reputation.
- Reputational risks. Customer trust is falling (“you’re spamming”), and the brand is suffering: support is inundated with tickets, and sales are filled with questions like “why isn’t it working?”.
How to Check If an IP Is Blacklisted
Online verification (fast and clear).Search for “IP blacklist check” and run your IP through multi-checkers (they check dozens of DNSBL and mailing lists). It is useful to look at both the general reputation (IP/ASN/geo) and specific DNSBL matches.
Checking from the command line.
Find out your public IP:
- Windows: nslookup myip.opendns.com resolver1.opendns.com
- macOS/Linux: dig +short myip.opendns.com @resolver1.opendns.com
- Manual verification in DNSBL (example for Spamhaus ZEN; IP 1.2.3.4 needs to be deployed in 4.3.2.1):
- dig +short 4.3.2.1.zen.spamhaus.org TXT (macOS/Linux)
- nslookup -q=TXT 4.3.2.1.zen.spamhaus.org (Windows)ю If the TXT response is returned, the IP is most likely in the list. Empty — with a probability of approx.
Frequent captchas, login refusals, massive email bonuses, a drop in the Open Rate, “suddenly” blocked forms/comments — all these are signals to check the IP reputation.
Removal and Prevention of IP Blacklisting
How to remove an IP from the blacklist (delisting).
- Identify the cause. The logs of the mailer, web server, and WAF/IDS will show spam spikes, brute force, scan, or virus traffic.
- Fix the root cause. Close open-relay, update CMS/plugins, disable compromised keys/API, enable rate-limit/captcha, run antivirus/anti-malware.
- Submit a delisting request. There are withdrawal forms on DNSBL websites. Describe what has been fixed, attach the evidence (logs, configs).
- Confirm the corrections with time. Many lists do not remove the block immediately, but after a period of “pure” activity.
To whom to write
- Provider/hoster. If the IP is shared (NAT, hosting), ask for a change of address or help with delisting.
- The service administrator. If the block is local (on a specific site/mail gateway), ask to remove the block after the reasons have been eliminated.
Prevention (so as not to return)
- Mail: SPF, DKIM, DMARC; double subscription confirmation; database cleaning; sending limits; honest From/Return-Path; warm IP (warming up).
- Security: OS updates/SOFTWARE; WAF/IDS/IPS; fail2ban/bruteforce-limits; HTTPS everywhere; network segmentation; monitoring logs and alerts.
- Proxy/VPN: use reliable residential/mobile IP, dedicated if possible; avoid public/shared proxies; monitor the consistency of IP, DNS, WebRTC.
- Processes: uniform password rules and 2FA; access control; regular audits.
If you want to remove your IP from blacklist fast feel free to check our article about it.
IP Blacklist Conclusion
IP blacklist is not a verdict, but a symptom. He says: the rules, safety or hygiene of sending and traffic have been violated somewhere. A quick action plan: confirm the problem (online checkers + DNSBL), find the root cause (logs, monitoring, antivirus), fix it (patches, limits, captcha, mail policies), then file for delisting and keep a clean reputation: regular updates, secure email practices, careful work with proxies/VPNs. This way you will regain access to the services, restore the deliverability of emails and maintain customer trust.