I have read this updated. However, I wasn't sure if the change I make to this line will effect anything else.
This was suggested by Justin's article at taint.org
Article link taint.org: Justin Mason’s Weblog » Dealing with backscatter, revisited
header_checks = regexp:/etc/postfix/header_checks
Our current main.cf file contain the following syntax
header_checks = pcre:/opt/zimbra/conf/postfix_header_checks
What is your though on this?