apt-get install postfix-policyd-spf-python

edit your /etc/postfix/master.cf

policyd-spf  unix  -       n       n       -       0       spawn
    user=policyd-spf argv=/usr/bin/policyd-spf

edit your /etc/postfix/main.cf

policyd-spf_time_limit = 3600
smtpd_recipient_restrictions =
...
 reject_invalid_hostname,
 reject_unauth_destination,
 reject_rbl_client cbl.abuseat.org,
 reject_rbl_client b.barracudacentral.org,
 reject_rbl_client zen.spamhaus.org,
 check_policy_service unix:private/policyd-spf,
...

Make sure, you are putting the check_policy_service unix after the reject_unauth_destination line, to not become an open relay to the world.

/etc/init.d/postfix restart

edit /etc/postfix-policyd-spf-python/policyd-spf.conf and set

HELO_reject False

Mail_From_reject False

this will accept Emails with wrong SPF but SA will mark them as spam afterwards.