sendmail_path = /usr/sbin/sendmail -t -i -f email@example.com
You can also add it to a vhost.conf (php_admin_value)
However, if the script uses its own smtp server/connects directly to a socket, then this won't make any difference. In such cases there would always be an option to change who the email is from.
Ok after lots of hours and a lot of nerves I found out how to solve the issue.
Both -f firstname.lastname@example.org
and (php_admin_value) did not solvethe problem !
Neither around 20 other workarounds I found on the net.
So I removed exim and I installed postfix.
It worked right away like a charm with the -f flag and the email set to it.
Did some testing and up to know it works perfectly.
Postfix is legendary.