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Help Docs Email Spam Management What Is an RBL?

What Is an RBL?

An RBL (remote block list or realtime blackhole list) is a spam-fighting tool that lists spammy IP addresses, affecting both incoming and outgoing emails.
Spam
3 min read

An RBL (remote block list or realtime blackhole list), also called a DNS-based blacklist or block list, is a spam-fighting tool. At Liquid Web, we use the general term RBL. RBLs work by keeping lists of IP addresses that are linked to spam email behaviors.

Each block list has different criteria for listing IP addresses as “spam” IPs. Different organizations maintain RBLs: there are public RBLs like SpamHaus that anyone can use, but there are also private RBLs used by service providers such as AT&T and Google. Online RBL checkers can’t check your IP address against these private lists, but you will usually get a bounceback message when you try to send email to that provider. Liquid Web does not control or manage any RBLs, buy you can read our article on troubleshooting RBLs.

There are two ways RBLs can affect your email: by preventing spam from reaching your mailbox and by preventing email you send from reaching other mailboxes.

Using RBLs for spam filtering is great for your email inbox. If your mail server uses RBLs for spam filtering, it checks all incoming mail against RBLs to determine which mail to deliver to you. Essentially, your server asks an RBL if the email sender IP is on its list of spam addresses. If it is, your server can choose to: reject the message and send it back with an error, accept the message and tag it for further spam filtering, or accept the message and deliver it to your inbox. This happens before your email even reaches your mailbox. This means there are fewer messages for your mailbox’s spam filter to analyze, helping mail delivery.

This spam-fighting power can work against you if your IP gets listed on an RBL. There are a few different ways this can happen.

  • An email account on your server could be compromised. This means someone has gained access to an email account and is sending spam from that email address.
  • A spammer is sending email that looks like it’s coming from a domain on your server. This is called email spoofing.
  • An email account is forwarding all their email, including any received spam, to a different address on a different server (e.g., forwarding your email to a Gmail account).
  • An email account on your server is intentionally sending spam.

If your IP address is listed on an RBL, you might have trouble sending legitimate email and will get error messages. If you see that your email is being rejected, it’s easy to check if your server’s IP is listed on any RBLs by using an online RBL checker. Remember that RBL usage is set by the receiver, not the email sender. This means you don’t have control over the RBLs your email recipients are using to filter their email. People can usually request to have their IPs delisted from RBLs, and some lists delist IPs automatically after a period of time.

Each RBL website will have instructions on how to request delisting. It’s important to solve the underlying email problem before you request your IP be unblocked. This can involve changing passwords, Adding a Sender Policy Framework (SPF) Record and Enabling DomainKeys Identified Mail (DKIM), reconfiguring your email forwarding, or speaking to email users on your server. If you request delisting but your server still appears to be sending spam, your IP will be relisted and it will be more difficult to delist a second time.

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