$arr = array(
    'my.mail.tw', 
    'my.nur.kz',
    'm.my.mail.ru', 
    'mirtesen.com', 
    'm.my.mail2.ru',
    'm.my.mail2.af',
);

function domenSort( $a, $b ) {
    $a = array_pop( explode( '.', $a ) );
    $b = array_pop( explode( '.', $b ) );
    if($a == $b ) {
        return 0;
    } return ( $a < $b ) ? -1 : 1;
}

echo usort( $arr, 'domenSort' );
print_r($arr);