While I don't know what php might be doing, I can guess that
it's a commandline "-f" option being used:
On 20/01/15 05:25, LinuxAdmin wrote:
> mail($to, "The subject", "The mail body", $header, -f returns@??? );
^^^^^^^^^^^^^^^^^^
and there's some slight correlation between this and exim's log line:
> In log file i am getting F=<returns@???> insted of actual From: address.
^^^^^^^^^^^^^^^^^^^^^
You might need to understand the difference between envelope and header.
--
Cheers,
Jeremy