Refused by block lists

[joe a]

Attempting to utilize the various block lists and find rejection 
messages in mail headers "blocked due to usage of an open resolver".

One of many things puzzling me at the moment is something found in the 
related Wiki that states "A: Third, if your email gateway is behind a 
firewall make sure that SpamAssassin is resolving the gateway to its 
external address."

I brazenly confess I have no idea how to check this (or what it means, 
in this context).

Figured I should sort out that puzzlement before attempting to install 
and configure "unbound" for example.