$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!";
}

$to = "email@address.com"; //where the email will go 
$subject = "Our Subject"; //the subject 
$message = "The message to send"; //the message 
$from = "you@email.com"; //from email 
$header = "From: $from"; //header including the from email

mail($to, $subject, $message, $header); //send the email

echo (mail($to, $subject,$message, $header)) ? "Email sent" : "Error sending email" ;

$header = "MIME-Version: 1.0" . "\r\n"; 
$header .= "Content-type:text/html;charset=iso-8859-1" . "\r\n"; //add content type 
$header .= "From: $from"; //header including the from email

function mailit ($to,$subject,$message,$from){ 
$headers = "MIME-Version: 1.0" . "\r\n"; 
$headers .= "Content-type:text/html;charset=iso-8859-1" . "\r\n"; 
$headers .= "From: <$from>" . "\r\n"; 
return (mail($to, $subject,$message, $headers)) ? TRUE : FALSE; 
}

$to = "email@address.com"; //where the email will go 
$subject = "Our Subject"; //the subject 
$message = "<strong>Message</strong>The html message"; //the message 
$from = "you@email.com"; //from email 
echo (mailit($to,$subject,$message,$from )==FALSE) ? "Sending Failed!" : "Mail sent!" ;