I had the same issue..but found this fix and it worked for me.
Go to the (class.misc.php) file..scroll down untill you reach the following lines below...
$eol="\n";
$fromname=$fromname?$fromname:$fromaddress;
//do some cleanup...avoid stupid errors.
$to=preg_replace("/(\r\n|\r|\n)/s",'', trim($to));
$subject=preg_replace("/(\r\n|\r|\n)/s",'', trim($subject));
$message = preg_replace("/(\r\n|\r)/s", "\n", trim($message));
#Headers
//$headers .= "From: ".$fromname."<".$fromaddress.">".$eol;
//$headers .= "Reply-To: ".$fromname."<".$fromaddress.">".$eol;
//$headers .= "Return-Path: ".$fromname."<".$fromaddress.">".$eol;
//$headers .= "Message-ID: <".time()."-".$fromaddress.">".$eol;
//$headers .= "X-Mailer: osTicket v 1.6".$eol;
if($xheaders) { //possibly attachments...does mess with content type
$headers .= $xheaders;
}else{
//$headers .= "Content-Type: text/plain; charset=utf-8".$eol;
//$headers .= "Content-Transfer-Encoding: 8bit".$eol;
Put // in front of the lines you see above and hit (save). It worked for me.