Reject / Discard / Redirect emails based on header checks :

vi /etc/postfix/main.cf header_checks = regexp:/etc/postfix/header_checks vi /etc/postfix/header_checks /^From: "Mechanic/ REJECT /^To: someone@example.com/ REDIRECT someone@example2.com /^To: [bademail@example.com/ REJECT /^Subject](mailto:bademail@example.com/ REJECT/^Subject): .*badWord/ DISCARD

Reject By Subnet Range (CIDR)

Add  to the recepient restriction in the main postfix configuration file :

vi /etc/postfix/main.cf smtpd_recipient_restrictions = check_client_access cidr:/etc/postfix/block.cidr

Now create the block.cidr file and add any ip ranges in the CIDR format that you may want to block :

vi /etc/postfix/block.cidr 192.168.0/24 REJECT

Note : The postfix needs to be reloaded in order to re-read the changes in block.cidr file