Well, there's nothing much really to say...

Just make a new file and name it "fakemailer.php", then paste this into it.

PHP Code:
<?php
$to
="enter target email address";
$subject="Enter subject";
$msg="Enter message. Use \n for line indentations.\n";
$msg.="This is the second line in the message.\n";
$msg.="This is the third line in the message";
$from="Enter the domain you want to impersonate e.g. support@steam.com";
$header="From: ".$from//ignore this
$r=mail($to,$subject,$msg,$header);
if (
$r==true) {
  echo 
"Mail successfully sent";
} else {
  echo 
"Delivery failed";
}
?>
You gotta edit the code each time and change the $from="blblabla"; and shit.

If anyone wants, they can re-release this with a GUI.