Op de site;
https://spamhaus.org kun je meer informatie vinden.
Dit stukje vond ik in ieder geval;
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This IP address range has been identified by Spamhaus as not meeting our policy for IP addresses permitted to deliver unauthenticated 'direct-to-mx' email to PBL users.
Important: If you are using any normal email software (such as Outlook, Entourage, Thunderbird, Apple Mail, etc.) and you are being blocked by this Spamhaus PBL listing when you try to send email, the reason is simply that you need to
turn on "SMTP Authentication" in your email program settings. For help with SMTP Authentication or ways to quickly fix this problem;
https://www.spamhaus.org/faq/section/Spamhaus%20PBL------------------------------------------------------------------------