Each restriction list is evaluated from left to right until
some restriction produces a result of PERMIT, REJECT or DEFER (try
again later). The end of the list is equivalent to a PERMIT result.
By placing a PERMIT restriction before a REJECT restriction you
can make exceptions for specific clients or users. This is called
whitelisting; the last example above allows mail from local networks
but otherwise rejects mail to arbitrary destinations.
The table below summarizes the purpose of each SMTP access
restriction list. All lists use the exact same syntax; they differ
only in the time of evaluation and in the effect of a REJECT or