How to rewrite headers in outgoing mail messages

Warning: You must make sure that the postfix-pcre package is installed on your Linux server before following the steps below, because if it is not, the pcre (Perl Compatible Regular Expressions) rules we need to add to the /etc/postfix/header_checks will not be interpreted correctly by the postfix mail server and that will cause mail delivery issues.

As a workaround, use Postfix header_checks:

1. Connect to the server via SSH

2. Add a header_checks entry to the /etc/postfix/main.cf configuration file (the file specified will contain regular expressions for the rewriting):

   # postconf -e "header_checks = pcre:/etc/postfix/header_checks"

3. If the file /etc/postfix/header_checks does not exist, create it:

   # [ ! -f /etc/postfix/header_checks ] && >/etc/postfix/header_checks

4. Modify the /etc/postfix/header_checks file:

   • For example, to rewrite all headers with From: [email protected] to From: "Mailer-Daemon at server.example.com" <[email protected]> , add the following line to the end of the file:

     /^From:[email protected]/ REPLACE From: "Mailer-Daemon at server.example.com" [email protected]

   • To hide server IP address from headers:

     /^Received:s+.*/ IGNORE

     Note: For more information on regular expressions, refer to the PCRE documentation by php.net .

   • To change Message-ID:

     /Message-Id:s+<(.*?)@server.example.com>/ REPLACE Message-Id: [email protected]

5. Reload Postfix to apply the enabled header_checks:

   # service postfix reload

If a regular expression was applied to the mail message, a similar line will be appended to the /var/log/maillog log file:

server postfix/cleanup[21124]: E5CD0225EF9: replace: header From: [email protected] (Mail Delivery System) from local; from=<> to= [email protected]: From: "Mailer-Daemon at server.example.com" [email protected]

Note: For more information on header_checks feature in Postfix, refer to the Postfix documentation .