If you configure your mail servers to use the PBL you can block large volumes of spam from entering your network, with almost zero false positives.
The PBL is maintained by Spamhaus researchers and contains approximately a quarter of IPv4 IP address ranges, numbering around 1 billion IP addresses.
While some individual IP addresses are included, most PBL listings are in classless inter domain routing (CIDR) format and are at least /24 in size.
If you are a mail server administrator working for an ISP or large enterprise, you can sign up for an account and manage PBL listings for your own CIDR IP ranges.
By managing your own IP address range on the PBL, your organization can protect other networks from receiving spam from infected devices on your network. This helps to protect email recipients from malware, preserves the reputation of your company and avoids your domain being added to a DNSBL, which would result in your organization’s outgoing emails being blocked.
You can configure your mail server to handle connections from IP address ranges listed in Spamhaus PBL in any of the following ways:
- Refuse connection and reject delivery (recommended for most mail servers)
- Accept email but silently drop it, or save it into a system spam folder
- Accept the connection and mark it as probable *SPAM* before delivery to recipient
- Tarpit emails from PBL-listed IP address ranges
Please note that when configuring your server to use PBL, you must ensure that you exempt your own IP ranges, in order to prevent your mail servers from blocking email from your own users.
If you are operating an environment where outright rejection of email is not practical, you can use PBL in conjunction with anti-spam filters that operate a scoring technique, such as SpamAssassin.
You should only use PBL to check IP addresses that directly connect to your mail servers. If you check IP addresses from other Received headers, you will block email sent by end users through designated IP mail severs or a legitimate third party relay. This will result in significant amounts of legitimate emails being blocked, while catching very little extra spam.
Spamhaus researchers manually check to verify that legitimate SMTP servers have not been included within the PBL. In case a legitimate server is inadvertently included, PBL also has a self-removal facility. This is monitored to prevent the delisting facility being abused by spammers.
PBL makes up part of the Spamhaus ZEN composite blocklist, comprising SBL, XBL and PBL.
1 BBC: “Fridge sends spam emails as attack hits smart gadgets,” 17th January 2014 http://www.bbc.co.uk/news/technology-25780908
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