Prevent an unjustified real-time blackhole listing (RBL)

Prevent an unjustified real-time blackhole listing (RBL)

Since the rise of the internet, one phenomenon has grown alongside it: spam. Everyone deals with it—unwanted emails flooding their inboxes. Many email clients have built-in features to filter spam from incoming emails. Internet Service Providers (ISPs) also play their part by using Real-time Blackhole Lists (RBLs). These are broad nets cast against spam, and you definitely don’t want your server to get caught in one. To avoid this, both ISPs and users can take preventive measures.

Blackhole Lists

Spam is often sent automatically through infected computers, which are part of a victim’s network. Spambots on these infected machines send out massive amounts of unwanted emails. This traffic is detected, and the associated IP addresses end up on an RBL. Various organizations, such as SORBS, SpamCop, and UCE-Protect, manage these RBLs. Data centers, ISPs, and hosting providers use these “blacklists” to automatically block emails from listed IP addresses, ensuring such emails never reach users. However, data centers, ISPs, and hosting providers can also end up on these lists—and that’s the last thing you want.

How to Avoid Landing on an RBL

For data centers, ISPs, and hosting providers, having an active abuse policy is crucial to avoid being listed on an RBL. Start by implementing rate limiting on your SMTP server to prevent extreme spikes in email traffic. Additionally, ensure that your ISP scans outgoing emails for viruses and spam. This helps quickly detect any misuse originating from your server. If, despite your efforts, your ISP gets listed, you’ll want to know immediately. Actively monitor RBLs so you can take swift action if a listing occurs. This involves contacting the relevant RBL to request a delisting.

When a User Causes a Listing

Users can also be the reason for a listing. It’s important for users to keep their workstations up to date. Systems with outdated software can quickly become targets for malicious spambots. While deliberate spamming by a user will obviously raise red flags, even sending large volumes of emails in a short time—such as bulk mailings—can result in an RBL listing. Therefore, it’s essential to educate users on safe email practices.

The following tip might seem obvious, but many users still fail to meet this requirement: install a virus scanner for your email client. If a virus is sent via email—often without the victim even realizing it—the user’s mail server can end up on an RBL. Users with their own mail servers should also ensure these servers are scanned for spam and viruses. Limiting the number of email clients on the network can help too; only send emails through your own mail server. For users working with mailing lists, it’s critical to keep the mailing list software up to date to prevent vulnerabilities.

Dealing with Incorrect Listings

Due to the aggressive global fight against spam, it’s possible for an IP address to be incorrectly listed on an RBL. Most RBL services offer the option to appeal a listing, usually through their websites. They will then reassess whether the IP address was justifiably listed. However, some lists handle these requests slowly. In the meantime, users can temporarily configure their email to use a different SMTP server to continue sending emails.