I'm not 100% sure if this effects the email content, but as far as I know it does effect the sending and receiving of the email. The CRLF is not correct in the $headers variable as some backslashes are missing.
Also taking in consideration what NogDog said about checking if the DB entry exists as well.
$username = "";
$password = "";
$hostname = "";
$dbconn = mysql_connect($hostname, $username, $password) or die("Unable to connect to MySQL");
$database = mysql_select_db("email",$dbconn);
$query = "SELECT email FROM eMail WHERE eMail_ID = '" . $_POST["eMail_ID"] . "'";
$result = mysql_query($query) or die(mysql_error());
$data = mysql_fetch_assoc($result);
$entry = mysql_num_rows($result);
//Check if entry exists
if($entry !== 0){
$message = $data['email'];
$to = "email address" ;
$from = "email address";
$Subject = "Subject";
$headers = "MIME-Version: 1.0\r\n";
$headers .= "Content-type: text/html; charset=iso-885n-l\r\n";
$headers .= "From: $from\r\n";
if(mail($to, $from, $subject, $message, $headers)){
echo 'Email sent';
}else{
echo 'Failed sending email';
}
}else{
echo 'No email entry found';
}
mysql_close($dbconn);