Funcion mail() y acentos y eñes

Hola a tod@s!!! Resulta que tengo una pagina montada con php y el unico problema que tengo es que cuando la gente utiliza el formulario de contacto me llega el correo perfectamente pero no me pone las ñ ni los acentos (asi como caracteres especiales como arrobas, etc...)

He buscado mucho por internet y se que es un problema de la funcion mail, pero no se como solucionarlo. Os dejo el codigo del formulario a ver si me podeis hechar una mano. Gracias de antemano.

Código:

if (!defined('MODULE_FILE')) {
	die ("You can't access this file directly...");
}

require_once("mainfile.php");
$module_name = basename(dirname(__FILE__));
get_lang($module_name);

/**********************************/
/* Configuration                  */
/*                                */
/* You can change this:           */
/* $index = 0; (right side off)   */
/**********************************/
$index = 1;
$subject = "$sitename "._FEEDBACK."";
/**********************************/

include("header.php");
$cookie[0] = intval($cookie[0]);
if ($cookie[1] != "") {
	$row = $db->sql_fetchrow($db->sql_query("SELECT name, username, user_email FROM ".$user_prefix."_users WHERE user_id='$cookie[0]'"));
	if ($row['name'] != "") {
		$sender_name = $row['name'];
	} else {
		$sender_name = $row['username'];
	}
	$sender_email = $row['user_email'];
}

$form_block = "
    <center><font class=\"title\"><b>XATICAP - Asociación Xàtiva Carreras Populares</b></font>
    <br><br><font class=\"content\">"._FEEDBACKNOTE."</font>
    <FORM METHOD=\"post\" ACTION=\"modules.php?name=$module_name\">
    <P><strong>"._YOURNAME.":</strong><br>
    <INPUT type=\"text\" NAME=\"sender_name\" VALUE=\"$sender_name\" SIZE=30></p>
    <P><strong>"._YOUREMAIL.":</strong><br>
    <INPUT type=\"text\" NAME=\"sender_email\" VALUE=\"$sender_email\" SIZE=30></p>
    <P><strong>"._MESSAGE.":</strong><br>
    <TEXTAREA NAME=\"message\" COLS=70 ROWS=15 WRAP=virtual>$message</TEXTAREA></p>
    <INPUT type=\"hidden\" name=\"opi\" value=\"ds\">
    <P><INPUT TYPE=\"submit\" NAME=\"submit\" VALUE=\""._SEND."\"></p>
    </FORM></center>
";

OpenTable();
if ($opi != "ds") {
	echo "$form_block";
} elseif ($opi == "ds") {
	if ($sender_name == "") {
		$name_err = "<center><font class=\"option\"><b><i>"._FBENTERNAME."</i></b></font></center><br>";
		$send = "no";
	}
	if ($sender_email == "") {
		$email_err = "<center><font class=\"option\"><b><i>"._FBENTEREMAIL."</i></b></font></center><br>";
		$send = "no";
	}
	if ($message == "") {
		$message_err = "<center><font class=\"option\"><b><i>"._FBENTERMESSAGE."</i></b></font></center><br>";
		$send = "no";
	}

	if ($send != "no") {

	 	$sender_name = $sender_name;
		$sender_email = $sender_email;
		$msg = "mipaginaweb.com\n\n";
		$msg .= ""._SENDERNAME.": $sender_name\n";
		$msg .= ""._SENDEREMAIL.": $sender_email\n";
		$msg .= ""._MESSAGE.": $message\n\n";
		$to = $adminmail;
		$mailheaders = "From: $sender_name <$sender_email>\n";
		$mailheaders .= "Reply-To: $sender_email\n\n";

		mail($to, $subject, $msg, $mailheaders);
		echo "<P><center>"._FBMAILSENT."</center></p>";
		echo "<P><center>"._FBTHANKSFORCONTACT."</center></p>";
	} 




	elseif ($send == "no") {
		OpenTable2();
		echo "$name_err";
		echo "$email_err";
		echo "$message_err";
		CloseTable2();
		echo "<br><br>";
		echo "$form_block";
	}
}

CloseTable();
include("footer.php");

?>

<?php
if (!defined('MODULE_FILE')) {
die ("You can't access this file directly...");
}
require_once("mainfile.php");
$module_name = basename(dirname(__FILE__));
get_lang($module_name);
/**********************************/
/* Configuration */
/* */
/* You can change this: */
/* $index = 0; (right side off) */
/**********************************/
$index = 1;
$subject = "$sitename "._FEEDBACK."";
/**********************************/
include("header.php");
$cookie[0] = intval($cookie[0]);
if ($cookie[1] != "") {
$row = $db->sql_fetchrow($db->sql_query("SELECT name, username, user_email FROM ".$user_prefix."_users WHERE user_id='$cookie[0]'"));
if ($row['name'] != "") {
$sender_name = $row['name'];
} else {
$sender_name = $row['username'];
}
$sender_email = $row['user_email'];
}
$form_block = "
<center>XATICAP - Asociación Xàtiva Carreras Populares
"._FEEDBACKNOTE."
<FORM METHOD=\"post\" ACTION=\"modules.php?name=$module_name\">
"._YOURNAME.":
<INPUT type=\"text\" NAME=\"sender_name\" VALUE=\"$sender_name\" SIZE=30>
"._YOUREMAIL.":
<INPUT type=\"text\" NAME=\"sender_email\" VALUE=\"$sender_email\" SIZE=30>
"._MESSAGE.":
<TEXTAREA NAME=\"message\" COLS=70 ROWS=15 WRAP=virtual>$message</TEXTAREA>
<INPUT type=\"hidden\" name=\"opi\" value=\"ds\">
<INPUT TYPE=\"submit\" NAME=\"submit\" VALUE=\""._SEND."\">
</FORM></center>
";
OpenTable();
if ($opi != "ds") {
echo "$form_block";
} elseif ($opi == "ds") {
if ($sender_name == "") {
$name_err = "<center>"._FBENTERNAME."</center> ";
$send = "no";
}
if ($sender_email == "") {
$email_err = "<center>"._FBENTEREMAIL."</center> ";
$send = "no";
}
if ($message == "") {
$message_err = "<center>"._FBENTERMESSAGE."</center> ";
$send = "no";
}
if ($send != "no") {
$sender_name = $sender_name;
$sender_email = $sender_email;
$msg = "mipaginaweb.com\n\n";
$msg .= ""._SENDERNAME.": $sender_name\n";
$msg .= ""._SENDEREMAIL.": $sender_email\n";
$msg .= ""._MESSAGE.": $message\n\n";
$to = $adminmail;
$mailheaders = "From: $sender_name <$sender_email>\n";
$mailheaders .= "Reply-To: $sender_email\n";
$mailheaders .= "Content-type: text/html; charset=iso-8859-1 \r\n\n";
mail($to, $subject, utf8_decode($msg), $mailheaders);
echo "<center>"._FBMAILSENT."</center>";
echo "<center>"._FBTHANKSFORCONTACT."</center>";
}
elseif ($send == "no") {
OpenTable2();
echo "$name_err";
echo "$email_err";
echo "$message_err";
CloseTable2();
echo " ";
echo "$form_block";
}
}
CloseTable();
include("footer.php");
?>