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
