Hello,
You can edit get_email_headers() function within 'includes/common_functions.php' and add after this line:
$mail_headers .= ($mail_type) ? $eol . "Content-Type: text/html": $eol . "Content-Type: text/plain";
your charset:
$mail_headers .= "; charset=\"windows-1251\"". $eol;
Thanks,
ViArt Support Team