Spam Protection – How Do RBL Lists Work?

Smarthost

hat Are RBL Lists?

An RBL (Real-time Blackhole List) is a real-time updated list of IP addresses or domains that are considered suspicious, untrusted, or involved in sending spam. These lists are widely used in the fight against spam as they provide information that allows spam messages to be blocked before they reach an email inbox.

RBL lists operate independently of email service providers, and there are many of them. Some of these lists have a good reputation, making them important and widely used.

What RBL Lists Do We Use?

On our servers, we use several well-reputed lists, which means that the IP addresses listed are considered reliable. The credibility of an RBL list is primarily evidenced by minimizing the rejection of legitimate emails.

  • SpamCom Blocking List (spamcop.net): An RBL from the world’s largest networking company, Cisco. This is an international project in which anyone can participate and help fight spam by submitting the headers of emails considered to be spam.
  • The Spamhaus Project – ZEN (spamhaus.org): ZEN is a combination of all the protections provided by The Spamhaus Project. It includes IP addresses of sites that:
    • Are used to send spam
    • Are infected with third-party exploits
    • Have a low reputation
  • Barracuda Reputation Block List (barracudacentral.org): An RBL belonging to Barracuda Networks, containing information about IP addresses of servers that send spam, enabling the mail server to filter them out from non-spam messages.
  • Smarthost RBL: Despite using external information, there was a need to create and use our own source on spammers. We created a proprietary RBL at Smarthost.au. It serves as an additional barrier between servers sending unwanted messages and email accounts hosted on our servers.

What If My Address Is on an RBL List?

RBL lists are designed to protect email from spam. However, sometimes our addresses may end up on various RBL lists for some reason. It’s important to be aware of the actions (or inactions) that can result in being added to an RBL.

The primary reason IP addresses get listed on an RBL is the sending of spam from unsecured contact forms, which we assist in combating as a hosting company. You can learn more about unsecured contact forms and how to secure them in our blog posts: Spam from Default Joomla Form and Adding reCAPTCHA in WordPress for the Contact Form 7 Plugin.

Another reason is sending emails without proper authorization (e.g., without SMTP authentication).

While RBL lists are excellent sources of information about spammers, occasionally, an address that shouldn’t be listed may be mistakenly included. To ensure proper email functionality, RBL lists should be monitored. If this occurs and a legitimate address is wrongly marked as a spam sender, it’s usually possible to remove it from the list. Typically, if the listing is a one-time, incidental occurrence, you can have the address removed. It’s advisable to do so, as leaving the address on an RBL can result in emails not being delivered to the recipient.

At Smarthost.au, our monitoring continuously checks the reputation of our servers and ensures that our IP addresses are not listed on any RBLs. We also constantly monitor the state of the emails sent from our servers to catch any spam that might be sent from them.

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