You have to send a header with your message to tell the email browser how to interpret the message.
Replace the "From: address" with "$header"
$header = "From: $senders_email\n";
$header .= "MIME-Version: 1.0\n";
$header .= "Mailer: FCG Browser-Based Applications\n";
$header .= "Content-Type: text/html; charset=\"iso-8859-1\"\n";
mail("destination","subject","message",$header);