<?php

function validar_email($mail){
    if(preg_match("#^[A-Za-z0-9\._+-]+[@]{1}[A-Za-z0-9\.|-|_]*[.]{1}[a-z]{2,5}$#",$mail)){
        return true;
    }
    return false;
}

// tests

$emails = array('luis@teste.com', '@t.com', 'exemplo@12.c', 'teste_12-3@subdominio1.dominio.com.br');

foreach ($emails as $email)
{
    echo $email . ' ';
    if (validar_email($email))
        echo "<span style='color: green'>Email valido!</span>";
    else
        echo "<span style='color: red'>Email invalido!</span>";
    echo "<br />";
}