Mail System
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... might block correctly addressed messages, especially such originating
or terminating in a gateway to a different mail system than SMTP.
Before implementing such a policy, a careful inventory should be done
...
... The "MAIL From: <>" address is used in error messages from the mail
system itself, e.g. when a legitimate mail relay is used and forwards
an error message back to the user. Refusing to receive such mail
...
... basis. Since most users will not have a strong opinion (except that
they want to avoid spam) the mail system should provide a system
default and give each user the ability to override or modify that.
In a UNIX ...
...
The mail system should be carefully studied, e.g. how it behaves when
one or more of the support systems needed for a specific method
...