Configure Postfix to only receive email from FORWARDING_EMAIL

[katefike]

Goal

• Only allow incoming emails from the FORWARDING_EMAIL
  https://www.postfix.org/header_checks.5.html
  https://serverfault.com/questions/780473/postfix-header-check-regex-does-not-match-issue

Solution

1. Add header_checks = regexp:/etc/postfix/header_checks to main.cf
2. Create the file /etc/postfix/header_checks
3. Add !/^From:.+@(?!dev\.kfike\.gmail\.com$)/ REJECT You are not authorized :( to header_checks
4. Restart postfix sudo systemctl restart postfix

DONE WHEN

• The MX rejects (rather than discards) any emails sent by anything other than the FORWARDING_EMAIL. The MX gives a reply like "User is not authorized :("