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
