Prevent form email injection...HOW?

This is my sendmail code, how would i change this to prevent bot's from injecting my code?

$to = 'email@domain.com;
$subject = $_POST['subject'];
$message = $_POST['message'];
$from = $_POST['from'];
$from_check = $_POST['from_check'];
$full_name = $_POST['full_name'];
$tel = $_POST['tel'];
  
$headers = "From: ". $_POST['from'] ."\r\n";  
$headers .= "Reply-To: ". $_POST['from'] ."\r\n";  

  $message = "\nInformation Request:   ".$message;
  $message .= "\nName:   ".$full_name;
  $message .= "\nTel Number:   ".$tel;
  $message .= "\nEmail address 2 (could be different):   ".$from_check;
  
if (!preg_match("/\w+([-+.]\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*/", $from)) {
   	
  echo "<h4>Sorry info here</h4>";
  echo "<h4><a href='javascript:history.back(1);'>Click here to return</a></h4>";
} 

elseif ($from_check <> $from) {
   	  echo "<h4>Info here.</h4>";
  echo "<h4><a href='javascript:history.back(1);'>Click here to return</a></h4>";
}
elseif(mail($to,$subject,$headers,$message)) {
  echo "<h4>$full_name</h4>";
  echo "<h4>Your email has been sent, we will reply as soon as possible.</h4>";
} else {
  echo "<h4>Cannot send email</h4>";

Code (markup):

Thanks Ian

 

use strip_tags() on all POSTed information