$email = $_POST['email']; //getting the posted email
//sanitizing the email using FILTER_SANITIZE_EMAIL
//removes all illegal e-mail characters from a string
$email=filter_var($email, FILTER_SANITIZE_EMAIL);
//validating the email using FILTER_VALIDATE_EMAIL
//validates e-mail returning true if valid and false if invalid
(filter_var($field, FILTER_VALIDATE_EMAIL)) ? TRUE : FALSE;

function check($email)  {
     $field=filter_var($field, FILTER_SANITIZE_EMAIL);
     return (filter_var($field, FILTER_VALIDATE_EMAIL)) ? TRUE : FALSE;
}

$email = $_POST['email'];
if (check($email)==TRUE){
//send email
}else{
echo "Invalid email!";
}