When a customer forwards a bounce that says "blocked using a blocklist", or your outbound mail suddenly lands in spam everywhere, the first question is simple: is our address listed, and where? You do not need a detailed per-list report to answer it, just a quick check of the lists that matter most.
The Email Blacklist Check queries the default set of DNS blocklists for a mail server IP address or a domain and gives a one-line verdict plus a row per list: listed, not listed, or unknown. It uses the same server-side engine as the MultiRBL Check and links to it for the full report with delisting links.
What is an email blacklist check?
In simple terms
Email blacklists (more precisely blocklists) are lists of IP addresses or domains associated with spam or abuse, published by independent organizations. Receiving mail servers consult them when a message arrives. A blacklist check asks those lists whether your address or domain is on them, so you know whether receivers are likely to reject or filter your mail.
Technical background
Blocklists are queried over DNS (DNSBL, RFC 5782). For an IPv4 address the octets are reversed and the list zone is appended: 198.51.100.7 checked on SpamCop becomes 7.100.51.198.bl.spamcop.net. Domain lists are queried as <domain>.<zone>. An answer inside 127.0.0.0/8 means listed, and the last octet is a return code that often identifies the reason. NXDOMAIN means not listed.
Many lists refuse queries from large public resolvers or above a free quota and answer with special error codes instead. A reliable checker therefore needs its own resolver and must distinguish "not listed" from "could not check".
Why use an email blacklist check?
- Fast triage of bounces that mention a blocklist.
- Before launch: check a new outbound IP or relay before sending production mail.
- After an incident: see whether a compromised account or form already caused listings.
- Routine checks for small mail servers that do not have reputation monitoring.
- Domain reputation: check whether a domain is on a domain blocklist.
Who should use it?
- Administrators of small and medium mail servers.
- Help-desk and MSP staff answering "are we blacklisted?" tickets.
- Web administrators whose sites send mail through the server's own IP.
- Anyone who needs a quick answer before deciding whether a deeper investigation is needed.
How the email blacklist check works
The Email Blacklist Check runs the server operation used by MultiRBL with the default list selection:
- The input is validated: a public IPv4 or IPv6 address or a public host/domain name. Private, loopback, CGNAT and documentation addresses (such as 192.0.2.0/24) and internal names are rejected.
- For a domain, the NetTools.tech server resolves up to two IPv4 addresses through the public resolvers 1.1.1.1 and 8.8.8.8 for use on IP lists; the domain itself is queried on domain lists.
- All blocklist queries go to the NetTools.tech server's own recursive resolver (Unbound), never to public resolvers, because many lists block those.
- The default lists are Spamhaus ZEN, Spamhaus DBL, Barracuda BRBL, SpamCop, UCEPROTECT Level 1, PSBL and Mailspike. Spamhaus is only queried when a Spamhaus Data Query Service key is configured on the server; Barracuda only once the NetTools resolver is registered with Barracuda. Otherwise those rows show Provider unavailable with the reason.
- Each answer is interpreted strictly: only an answer in 127.0.0.0/8 that is not a documented error code is "Listed"; only NXDOMAIN is "Not listed"; timeouts, refused queries, error codes and odd answers are reported as such.
- Each query has a 4-second timeout. The same target can be checked at most 10 times in 10 minutes across all users, and per-client rate limits apply.
How to use the NetTools.tech Email Blacklist Check
- Find the IP your mail actually leaves from: the first external hop in a sent message's headers, analyzed with the Email Header Analyzer, or your relay's public address.
- Open Email Blacklist Check.
- Enter the address or domain in Domain or IP.
- Press Run Email Blacklist Check.
- Read the verdict and the table. Follow Full MultiRBL report to see query names, delisting links and to add optional lists. Copy and Open in terminal (
/blacklist <target>) are also available.
Understanding the results
| Result | Meaning | What to look for |
|---|---|---|
| "Listed on N of M blocklists." | At least one list returned a listing | Which lists, and their codes |
| "Not listed on N blocklists; K could not be checked." | No listing found, some lists unknown | The unknown rows are not clean |
| "Not listed on N blocklists." | Every checked list answered NXDOMAIN | — |
| Addresses checked on IP lists | For a domain: the IPv4 addresses used | That these are your real sending IPs |
| Listed | Answer in 127.0.0.0/8 | Return code and reason in Details |
| Not listed | NXDOMAIN | — |
| Provider unavailable | Access not configured, refused, SERVFAIL or error code | Reason in Details |
| Timeout | No answer within the limit | Try again later |
| Lookup error | Unexpected or empty answer, or no IPv4 address for a domain | Status unknown |
| Unsupported query | The list does not accept this kind of target, e.g. IPv6 on an IPv4-only list | Not counted as checked |
| Details | Return codes and their meaning, TXT reason or diagnostic text | Spamhaus codes are translated (SBL, XBL, PBL…) |
Example: checking an outbound IP
Illustrative output for a hypothetical outbound address 203.0.113.45 (a documentation address used here for illustration; the real tool rejects documentation ranges and needs your server's public IP):
Listed on 1 of 6 blocklists.
Blocklist Result Details
Spamhaus ZEN Provider unavailable Requires a Spamhaus Data Query Service (DQS) key, which is not configured on this server.
Spamhaus DBL Unsupported query —
Barracuda BRBL Provider unavailable Requires the NetTools resolver to be registered with Barracuda, which has not been done yet.
SpamCop Listed 127.0.0.2
UCEPROTECT Level 1 Not listed —
PSBL (Passive Spam Block List) Not listed —
Mailspike Blacklist Not listed —
Full MultiRBL reportReading it: SpamCop lists the address, so some receivers will reject mail from it. Two lists could not be checked from this server, so the picture is incomplete; use the providers' own lookup pages for those. Spamhaus DBL is a domain list and does not apply to an IP, which is why the verdict counts 6 checked lists, not 7. Next steps: find the source of the spam (mail logs, compromised accounts, scripts sending mail), stop it, and let the SpamCop listing expire or follow its procedure.
Common use cases
- Responding to a bounce mentioning a blocklist.
- Checking a newly assigned VPS or cloud IP before using it for mail.
- Periodic manual checks of a small office mail server.
- Verifying a domain before using it in a newsletter or link shortener.
- First pass before a detailed MultiRBL Check with optional lists.
Troubleshooting
Everything shows "Not listed" but receivers still reject my mail
Many large mailbox providers use internal reputation data that is not published as a DNSBL, and the bounce may name a list not included here. Read the exact bounce text, check authentication with the SPF Lookup and DMARC Lookup, and check reverse DNS.
"is not a public address"
You entered a private, CGNAT or documentation address. Blocklists only track public IPs; use the address your mail leaves from on the internet.
"The hostname has no IPv4 address to check on IP blocklists"
The domain has no A record. Check the actual mail server IP directly.
Results differ from another checker
Other services may query through public resolvers (and get refused or error codes), use different lists, or run at a different time. A refused query displayed as "clean" elsewhere is a common reason for differences.
Common mistakes
- Checking the website's IP when mail is sent through a separate relay or provider.
- Treating unknown results as clean.
- Requesting delisting before fixing the cause.
- Panicking about a listing on a list no receiver of yours uses; check bounces to see which lists matter.
- Checking the visible From domain only, when IP reputation drives most rejections.
Limitations
- Only the default lists are queried; use MultiRBL to add DroneBL, SURBL and URIBL.
- Spamhaus and Barracuda depend on server configuration and may show as unavailable.
- Domains are checked on IP lists via at most two IPv4 addresses; IPv6 addresses of domains are not checked.
- Results are a snapshot; listings are added and removed continuously.
- The tool cannot see private reputation systems of mailbox providers.
Security & privacy
The IP or domain is sent to the NetTools.tech server; the blocklist operators receive a DNS query from the NetTools resolver containing the target, never your IP address. Results are not stored; the tool log keeps the tool name, the target and your shortened IP address for 30 days (see the privacy policy). Only blocklist zones from the configured list can be queried. No credentials are needed.
Related tools
- MultiRBL Check – choose lists and see full per-list details (guide).
- Reverse DNS Lookup – check the PTR record of your mail server IP (guide).
- MX Lookup – find mail server addresses (guide).
- Email Header Analyzer – find the IP a message was sent from (guide).
- Email Tools overview – how the email tools fit together.
Summary
An email blacklist check answers quickly whether a mail server address or domain is on common DNS blocklists. The NetTools.tech Email Blacklist Check queries the default lists through its own resolver, honors each provider's access rules and keeps unknown results separate from clean ones. For list selection, query details and delisting links, continue with the MultiRBL Check.
Frequently asked questions
How do I check if my mail server IP is blacklisted?
Enter the public IP address your server sends mail from into the Email Blacklist Check and run it. The tool queries the default set of DNS blocklists and shows listed, not listed or an unknown status for each one.
My IP is listed. What now?
Fix the cause first: a compromised account, an infected client, an open relay, a hacked web form or missing reverse DNS. Then follow the delisting procedure on that list's own website; the full MultiRBL report links to each provider's info and removal pages.
Does a listing on one list matter?
It depends on the list and on who uses it. Widely used lists affect delivery to many receivers immediately, while smaller lists may be used by few servers; the bounce messages you receive tell you which lists actually matter for your mail.
Why do some lists show unavailable?
Spamhaus requires a Data Query Service key and Barracuda requires a registered resolver, and the NetTools.tech server only queries them when that access is configured. Lists can also time out or refuse a query, and in all these cases the result is unknown rather than clean.
Can I check a domain instead of an IP?
Yes. A domain is checked by name on domain blocklists and, for IP blocklists, through up to two of its IPv4 addresses, which are shown as the addresses checked. For mail, the IP your server actually sends from is usually the more important check.
What is the difference to the MultiRBL Check?
Email Blacklist Check always runs the default list selection and gives a compact summary with one row per list. MultiRBL Check lets you pick the lists, including optional ones such as DroneBL, SURBL and URIBL, and shows query names, response times and delisting links.