Re: Null sender rewrite from a specified host.

[Wietse Venema]

To cut through the noise:

1 - Configure a Postfix smtpd process on an additional IP addresss
     or TCP port ***THAT ONLY THIS CLIENT WILL CONNECT TO.****

2 - Configure that smtpd process with an smtpd_command_filter that
     replaces "MAIL FROM:<>" with the preferred address.

     # Listen on port 1234
     /etc/postfix/master.cf:
      1234   inet ..   ..   ..   ..   ..   smtpd
          -o {smtpd_command_filter = pcre:/etc/postfix/command_filter}

     # Listen on IP address 1.2.3.4
     /etc/postfix/master.cf:
      1.2.3.4:smtp   inet ..   ..   ..   ..   ..   smtpd
          -o {smtpd_command_filter = pcre:/etc/postfix/command_filter}

     /etc/postfix/command_filter:
        /^(MAIL FROM:)\s*<>/  MAIL FROM:<u...@example.com>

That is sufficient to rewrite the null sender from ONLY THIS SMTP client.

      Wietse