Enable SPF in postfix and Bind9??

[cosme]

Hi

To apply SPF to control mails as well as spoofing
users and more.
- I have a server with postfix mail for local users which is
assigned to use SASL to prevent spoofing mails.
- I have a postfix mail sever face as internet gateway both cases
Debian.
and Bind9 the internet facing server and enable SPF to prevent
from outside can send emails to my local domain as if they were
internal users ??
spamassasin use and amavisd-new also
As might enable it ???