Mails sent in Greek (encoding issue)

Hi all,
i have the same problem with all my 3 wordpress sites, i am Greek, so all my visitors send mails in Greek, the problem is that in mails i get only that ????? ??? ???????? ??? ? , when in Latin everything is fine, is there a way to change the encoding and where exactly is that file??
In my other sites (not WP) i use this code and i have not problems with mails , from Win or Macs ,

function mail_utf8($to, $subject = '(No subject)', $message = '', $header = '')
	{$header_ = 'MIME-Version: 1.0' . "\r\n" . 'Content-type: text/plain; charset=UTF-8' . "\r\n";
  	mail($to, '=?UTF-8?B?'.base64_encode($subject).'?=', $message, $header_ . $header);}

Please get back to me because i am new to WP and all those sites are for clients… and they get no mails….