Prevent SpamHaus PBL (Policy Block List) blacklisting in web hosting servers
554 5.7.1 Service unavailable; Client host [203.0.113.XXX] blocked using zen.spamhaus.org; https://www.spamhaus.org/pbl/query/PBL289647
554 mysev mysmtp 203.0.113.XXX is on the Spamhaus PBL Please visit: http://www.spamhaus.org/query/bl?ip=203.0.113.XXX
If you’ve received one such error message, you are not alone. Mail bounces due to SpamHaus PBL blacklisting is a common issue faced by web hosts around the world.
Our engineers see these errors everyday in their role as Outsourced Support Specialists for web hosting providers. And we’ve seen perfectly legitimate business mails sent from clean servers tagged as spam by the SpamHaus Zen composite block list.
To make matters worse, de-listing an IP in SpamHaus PBL is at best a temporary fix, and at worst – impossible. Sr. Support Engineer Rai Dhaman explains:
“We see 2 kinds of PBL bounce errors. One where the email user’s laptop, mobile or residential IP is block listed, and the second where the web hosts’s server IP is listed. As we’re the hosting support team, we cannot de-list the email user’s residential IP. As for server IPs, even if we de-listed the IP, it often gets back on the block list.”
Over time, our engineers have found effective ways to delist and prevent blacklisting in SpamHaus PBL. To know how, we need to first look at what exactly is this “Policy Block List”.
What is SpamHaus Policy Block List (PBL)?
The PBL is a list of all IPs that are not likely to be legitimate mail servers hosted by mail service providers. Bobcares Support Engineer Sambhu Prakash explains:
“When you or me send a mail using a laptop, the mail first goes to an ISP’s (or hosting provider’s) mail server, and then to the recipient. In contrast, a virus infected home PC by-passes the mail server, and sends millions of spam directly to recipients. SpamHaus found that almost all mails coming from non-server IPs such as home PC, laptops, etc. are spam. PBL is a collection of all such residential IPs.”
To make the PBL, SpamHaus compiled a list of IPs that are assigned to cable operators, mobile service providers, and generally any IP that’s used by end-user devices such as PCs, laptops and mobiles.
So then why are clean mails blocked?
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Why does PBL block legitimate mails and clean servers?
Legitimate mails are blocked in two situations. One, when the mail user do not authenticate while sending mails, and two, when the mail server IP previously belonged to a residential connection.
“Mail servers recognize a valid user through a valid username and password.” says Sambhu Prakash, “When a setting called ‘SMTP authentication’ is not enabled in mail clients, the mail servers do not recognize the user. At this point, the PBL says the IP is a residential IP. So the mail server puts 2 and 2 together, and tags the mail as spam.”
The second case is when an ISP or hosting provider takes an IP that was previously a residential IP, and uses it for mail servers.
“Many data centers have both residential internet services and hosting services” Rai says. “When web hosting providers ask for a new IP block, they’re some times given IPs that were used in cable services before. PBL has these “residential IPs” in their list. So, if these IPs are used in mail servers, mails from these servers will bounce.”
So, what’s the solution?