Alright, thank you for time Shamim. I know you must be busy preparing this week release.
Here’s the full function I try to make work. As you can see I use a priority of 99 for my filter and I unset the new message button.

add_filter( 'fep_menu_buttons', 'fep_cus_fep_menu_buttons', 99 );
function fep_cus_fep_menu_buttons( $menu )
{
    unset( $menu['newmessage'] );
    $menu['messagebox']['priority'] = 1;
    $menu['announcements']['priority'] = 2;
    $menu['directory']['priority'] = 3;
    $menu['settings']['priority'] = 4;
    return $menu;
}