[Chuck S]

It is not really supported but you can try this edit here and see

in adm-user.php

find this

$subject = addslashes( $subject );
$letter = addslashes( $letter );
$from_email = "From: {$Globals['adminemail']}";

while ( list($euserid,$eusername,$email) = mysql_fetch_row($fusers) ) {
mail( $email, $subject, $letter, $from_email );
}

change it to this

$subject = addslashes( $subject );
$letter = addslashes( $letter );
$from_email = "{$Globals['adminemail']}";

$headers = "";
$newline = "\n";
$headers .= "From: {$from_email}$newline";
$headers .= "Sender: {$from_email}$newline";
$headers .= "Reply-to: {$from_email}$newline";
$headers .= "X-Mailer: Photopost$newline";
$headers .= "Return-Path: <{$from_email}>$newline";
$headers .= "Content-Type: text/html; charset=iso-8859-1$newline";

while ( list($euserid,$eusername,$email) = mysql_fetch_row($fusers) ) {
mail( $email, $subject, $letter,"From: $from_email{$newline}$headers");
}