Overview
PBL is a list of IP space that should not be sending email direct to MX. These are typically IP addresses assigned by ISPs to broadband and dial-up customers. Other types of IPs could be included in the PBL, but all IPs in the list should not be sending email. Being listed on the PBL does not mean you have done anything wrong, it just means you are using an IP that hasn't been designated for sending email.
Impact
This listing does not prevent you from sending emails unless you are not authenticated correctly from the mail server.
Mitigation process
To better understand the potential problems that caused the listing, review the PBL website, and PBL removal documentation. Ensure that you are using the proper SMTP Authentication when attempting to send traffic. Contact Mailgun if you experience this bounce. Only an ISP can submit a request for removal of listing.