sendmail and -f flag??

I've been having problems with mail() and I got this message from technical support for my ISP but don't understand what they mean:

You need to specify a Control Panel domain to sendmail with a -f flag in the mail command.

A control panel domain is simply one of my domains hosted by the ISP, but how do you specify a domain to sendmail with a -f flag in the mail command?

TIA

Julia

@mail("$sendto", $msubject , stripslashes($sendmsg), $headers, "-f$sentby");

The "-f" alters the sender of the mail.

without "-f" the "Sender" is wwwrun on the sending machine.

With -f the header of the outgoing mail contains a line, like this:

X-Authentication-Warning: sending.machine.com: wwwrun set sender to a.dummy@xyz.com using -f

So: just place one of your email-addresses behind the -f
e.g. "-fwebmaster@abc.com"

so is this right?

Julia

<?php
//create content for email

$email = "julia@myemail.com";
$subject = "test successful";
$message = "test worked, yippee";
$header = "From: -fwebmaster@$SERVER_NAME";

//send email
mail($email, $subject, $message, $header);

// confirm details
echo "email sent to: $email with subject: $subject";
echo "message body: $messagemessage from $header ";