Regarding $SenderMustExistInExternalDatabase

I’m guessing this is an anti-spam measure. Where does one configure the
external database and is it something for which queues can be excluded? I ask
because it would be neat to not have to deal with our current anti-spam method
and simply have emails accepted or rejected based on our already existing
customer database. On the same token, however, is our security queue to which
emails regarding abuse and other, necessarily publicly available purposes are
sent. This is one that would need to be excluded from such settings.

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