Hi Wieste,
> 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. Many thanks for this nice hint :) Xavier