Policy block list:
The Policy block list or PBL is a term used by anti-spam email blockers.
The PBL is a database of Internet end-user IP address ranges which should not be delivering unauthenticated SMTP email to any Internet mail server except those provided for specifically by an Internet Service Provider for that customer's use.
If an end-user is attempting to send email via a home SMTP email server (or potentially if they are infected with a spam program) then this can be detected.
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