Guides BLACKLIST
What is a blacklist (DNSBL)?
To stop spam, mail servers use blacklists: lists of IP addresses known for abuse. Such a list is also called a DNSBL (DNS-based Blocklist), because the check runs over DNS. If you send mail from an IP address on such a list, receivers distrust your mail: it lands in spam or is even refused outright. DNS Radar's Mailheaders tool can check the sending IPs from a mail against several of these lists.
How it works
When a mail server receives mail, it looks at the sending server's IP address and asks one or more blacklists: 'is this address known to you?'. That question goes over DNS, hence the name DNSBL. If the address is on the list, the mail gets a mark against it: more chance of the spam folder, or outright rejection.
Each blacklist has its own rules about who ends up on it and how strictly providers weigh it. One counts heavily, another barely. Besides IP lists there are also lists of domain names that show up in spam (like the Spamhaus DBL): the same idea, a different target.
Why you can end up on a list
- An infected computer or website on your network sends spam without you knowing
- A hacked mail account is abused to send mass mail
- You send too much mail at once yourself, or to many invalid addresses
- You share an IP address with others (on cheap hosting) and a 'neighbor' causes trouble
- A misconfigured mail server that can be abused as an 'open relay'
How to get off
- First find the cause: an infected machine, a hacked account or a wrong setting. If you don't remove it, you'll be back on the list in no time.
- Go to the relevant blacklist's website. Most have a 'delisting' form to request removal.
- Make sure your SPF, DKIM and DMARC are in order: that helps build your reputation.
- If you're on a shared IP and it's someone else's fault, get your hosting provider involved.
- Never pay for 'express delisting': serious lists remove you for free. Lists that charge money carry little weight with big providers and are usually safe to ignore.
Reputation is broader than blacklists
Not being on a blacklist doesn't automatically mean your mail arrives well. Large providers (Google, Microsoft) have their own, secret reputation systems that look at much more: how many people mark your mail as spam, whether your SPF/DKIM/DMARC check out, how consistently you send. Blacklists are one signal. A good sender reputation is built more broadly.
Frequently asked questions
How do I know if I'm on a blacklist?
Paste a sent mail into DNS Radar's Mailheaders tool and use the opt-in blacklist check. It tests the sending IPs against several lists. You can also look an IP address up directly on a list's own site (such as Spamhaus).
I'm on one but I don't spam. How come?
Often you share an IP with others, or something infected is running unnoticed on your network. Sometimes you've accidentally sent too aggressively. Find the cause before requesting delisting.
Do SPF/DKIM/DMARC help against blacklisting?
Indirectly: they prove your mail is genuinely yours and build reputation. They don't prevent blacklisting directly, but they're part of healthy sending practice.
How long am I on a list?
It varies per list: some remove you automatically after a period without abuse, others require a manual delisting request. Always fix the cause first.