Updated: 28 September 2026 · Applies to: Self-managed dedicated servers
An IP address has a history: earlier users may have run services on it, and public blocklists may still list it. Before you put an important service on a new address, and again from time to time, check what public lists say about it. This matters for mail servers, but also for websites, APIs and anything else that connects to other people's systems.
Check right after delivery
Do this as early as possible, ideally the same day. Under Ucartz's terms IP addresses are final after delivery, and changes are only made for specific reasons (renumbering by the network, compliance actions or abuse mitigation), so early information helps everybody.
- Note all addresses of the server and of any additional block (How many IP addresses come with a dedicated server and what do additional subnets cost?).
- Look each address up on a public blocklist checker, for example the Spamhaus IP and domain reputation checker. Check addresses one by one in the web form.
- Also read the result for the whole network block if the tool offers it: a listing of a range can affect every address in it.
- Look at the reverse DNS name (
dig -x IP +short). Names left from an earlier user are replaced when you set your own (How to set reverse DNS (PTR) for many IP addresses and check forward-confirmed rDNS?). - Write down what you found, with the date.
Do not run mass lookups yourself
Some blocklists offer their data through DNS queries, and they restrict heavy use or require registration. Use the web checkers for occasional checks. If you need automated checks for a large block, use a service or data feed with terms that allow it.
If an address is listed
- Read the reason on the blocklist's page. Different lists have different rules and different removal processes.
- If you have not used the address yet, do not start services that depend on a clean reputation (such as mail) on it. Open a support ticket with the address, the list name and the date, and ask what can be done. Whether a change is possible is decided by Ucartz.
- If the address was fine and became listed after you started using it, the cause is on your side: see What to do when your IP address is on a blocklist or your mail is rejected? for the steps to find and fix it.
Other things to check
- Geolocation: websites that show a visitor's country use databases that lag behind. The data centre's location is not a promise about what these databases say, and you are responsible for asking the database owners to update their records if it matters to you.
- Old reverse DNS or open ports: scan your own address from outside (
nmap -Pn IP) and close what you do not need.
Frequently asked questions
Does a listing always mean the address is bad?
No. Some lists cover whole ranges or list addresses for reasons that do not apply to your use, for example a list of dynamic addresses. Read the reason.
How often should I check?
At delivery, before you start a service that depends on reputation, and then regularly. For mail, watch the signals in How to monitor your sending IP addresses automatically with a script?.
Need a dedicated server, more IP addresses, or a hand with the setup?
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