The PBL (Policy Block List) is a centrally maintained list of IP spaces within which the owners of that space can set parameters for the user of that space. In our use of the PBL, we check to see if the owners of IP address space permit (or deny) the use of that space to originate email. For example; BT recently added their entire ADSL IP pool to the PBL and denied its use for originating email, as is their right. This means, that any attempt to send email from a BT ADSL will fail on servers which incorporate the PBL and that's ok because all outgoing email should always be routed via ISP's mail servers and not originated directly. 

For example; All users of GEN's IP Space are permitted to send email via mail.genzone.net and mail.genzone.net will accept that email regardless. But if a user of GENZone/GENX wishes to use BT's ADSL instead, how does GEN know to accept that email and not reject it based on the PBL? 

The answer is because when sending email, the users client will Authenticate with GENX/GENZone and therefore bypass the PBL (and all other blacklists/spam measures). 

So, if you find that when trying to send email and you have an account on GENX/GENZone, you get a rejection relating to a blacklisting, it means that your not using authentication. To turn it on is a simple affair and instructions are provided in the FAQ for some clients, others you can find easily by searching the help for 'outgoing authentication'. 

See Also General Information on Blacklists