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DNSBL
/ RBL Services
(DNS
BlackList
or DNS Blackhole
List
or Realtime
Blackhole
List)
A system for identifying spammers based on their IP addresses. Most
mail servers are able to query a DNSBL to determine if a specific IP
address is on the list.
DNSBL/RBL uses the DNS protocol to query the well-known IP addresses which have been complaint to send worm, virus & junk mail. Using RBLs to prevent junk, virus & worm email is very effective. The algorithm of RBLs is different with that of Anti-Spam and Anti-Virus services as RBLs works based on IP address, while Anti-Spam and Anti-Virus services work based on the message content.
The RBL-DNS (RBL) utilizes a standard DNSBL (Domain Name System Block List) implementation that can be used to reduce spam volume on many popular email systems. DNSBL systems list IP addresses, often those that have been observed by the list operator to be sending spam or hosting spammers. By adding DNSBL entries to an email server configuration, those email servers can either reject connection attempts from listed IP addresses or use that listing to apply appropriate filtering policy.
It is important to note that a DNSBL cannot stop anyone from sending email. A DNSBL only prevents delivery at the receiving end at the receiver's instruction. DNSBLs are strictly defensive tools, and they cannot do any offensive damage such as denial of service attacks.
An RBL blacklist, also known by several other names including DNSBL (Domain Name System Blacklist), Remote Block List (RBL), Realtime Blackhole List (RBL), or simply Spam Blacklist or Block List, is a constantly updated list of IP addresses that are known to send spam or other malicious content.
If an IP address is identified as a source of spam or malware, it gets added to an RBL blacklist. This can happen due to intentional spamming or even accidental configuration issues on a mail server.
When you send an email, your server's IP address is checked against these RBLs by the receiving server. If your IP is listed, your email might get rejected or marked as spam. being on an RBL blacklist can seriously affect your email deliverability, meaning your legitimate emails might not reach their intended recipients.
Spam is any unsolicited, irrelevant, or inappropriate email sent to a large number of recipients without their consent. For example, emails sent to recipients who have not opted-in or given explicit consent to receive communications from the sender can constitute spam.
If you’re a cold email marketer, avoiding your emails being marked as spam becomes even trickier. However, a few strategies can help you avoid being flagged by these blocklists (more on this later).
Getting your IP blacklisted can be due to several reasons. Here’re a few common reasons that can put your emails in the suspicious position:
- Complaints from Recipients: If a significant number of people who receive your emails mark them as spam, email providers consider it a strong indicator of unwanted emails. This can lead to your IP being blacklisted to protect their users from potential spammers.
- Poor Contact Database: Sending emails to invalid or inactive addresses is a red flag for email providers. These addresses might be old, misspelled, or simply non-existent. A high bounce rate (emails that fail to deliver) due to a poor contact list suggests spammy practices and could trigger blacklisting. Learn more about how list quality can affect your inbox placement.
- Spamtraps: These are strategically placed email addresses designed to catch spammers. They are typically never used by real people and only exist to collect spam emails. Sending emails to even a single spamtrap can land your IP on a blacklist because it's a clear sign of someone trying to distribute unsolicited emails.
- New IP: Email providers are more cautious about new IPs because they have no established reputation. To avoid suspicion, consistently send legitimate emails and build a positive sending history to establish trust with email providers.
- High Sending Volume: Sending a large number of emails in a short period can be mistaken for a spam campaign. Email providers might interpret this sudden surge as an attempt to distribute unwanted emails and blacklist your IP to protect their users from potential spam attacks.
The DNSBL / RBL can be queried directly as bl.rbl-dns.com
Currently, the following zones exist:
- bl.rbl-dns.com combination of spam.rbl-dns.com and dul.rbl-dns.com in a single zone.
- spam.rbl-dns.com (Open Relay, Hijacked PCs, Spam Source)
- dul.rbl-dns.com (Dynamic, ADSL, Cable, no PTR Networks)
Doing a DNSBL lookup on a message at SMTP connect time is cheap in hardware cycles and system time. Your DNS server may even have it cached from the last time the spammer tried.
Mail rejected by a DNSBL during delivery is not silently discarded or lost. A DNSBL realtime rejection creates a delivery status notification to the sender identifying the cause of the rejection, thereby allowing troubleshooting on the sender's end.
By rejecting the connection attempt in real-time, a DNSBL also avoids any problems associated with an email system accepting delivery, closing the connection, and then trying to return the mail to a potentially forged address after the message is determined to be spam.
DNS is inherently very efficient, using minimal amounts of bandwidth. Using BL will use much less bandwidth than having to accept every spam and virus email sent to your email system. By rejecting these spam messages during the SMTP connection, no further data is sent thereby reducing overall bandwidth requirements. DNS caching by your local DNS server also prevents redundant queries from utilizing excessive bandwidth.
The use of the RBL-DNS DNSBLs is free of charge for low-volume non-commercial use.
For professional use, internet ISPs, or hosting providers, we can provide your unlimited or dedicated realtime access to all RBL-DNS DNSBLs from your network.
Free mirror service is provided to sites who are willing to host a public mirror (serving data to other free users).