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Real-Time Blackhole List (RBL)

The Real-Time Blackhole List (RBL) is a dynamic list of IP addresses and owners who are known to be active spammers or spam sources. This list may also include Internet service providers (ISPs) with customers who are known spammers or even ISP servers which are hijacked for spamming purposes.

This list is also commonly referred to as the DNS blacklist DNSBL. RBL was created by Mail Abuse Prevention System LLC, but there are other entities that also keep RBL's.

What Are RBL's Used For?

In simple terms, RBL prevents system abuse by stopping spam. This is extremely helpful, especially when spam is so prevalent. Because most spam is as a result of online advertisements and unsolicited bulk email, it could however pose a problem for marketers. With this system, when spam is delivered, the forwarding IP address is added to the list.

There are many RBL servers with some being private, and others being public. These lists are populated in different ways. Some of these, for instance, create an email address called a spam trap. This is an active email address, but which is not in use. So, if an email arrives at this address, it's highly probable that it is spam, and the sending server will be registered on the list.

Manual flagging is also commonly used to add IP addresses to the list. Here, when someone marks an email as spam, their specific email server will take that into account. If, however, several people mark the same message as spam, the sending server will be listed on the RBL.

As subscribers to the RBL, ISPs and other companies know from which IP addresses to block traffic. To do this, most of the blocking occurs during the SMTP connection phase. 

Here, the receiving end of the message will check the RBL for the IP address and if the IP addresses matches one on the list, the connection gets dropped before accepting any email from spam server. 

Why are RBL's Important?

Because the RBL keeps emails free from spam, it plays an important role in the proper functioning of email. However, this can also pose a problem to businesses and marketers who rely on email in their businesses. 

For these businesses, it's important to ensure that they do not get listed on the RBL because this will affect their email deliverability. For this reason, it’s always advised to use the right warming up techniques before using an email address for marketing purposes. 

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