UNTESTED CODE - A simple unubtrusive captcha, that requires JS and Cookies to be enabled, as well make sure the form isn't submited before 3 seconds.

After Form

<?php


$expectedTimeField 		= md5($_SESSION['captcha_field']."_timer");

if 	(
			empty($_POST[$expectedTimeField]) // Timer field doesn't exist, fake it.
		|| 	!is_numeric($_POST[$expectedTimeField]) // Timer is non-numeric, JS didn't exec, fake it.
		|| ($_SESSION['captcha_time'] + 3000) < (time() + date("Z", time()) ) // They took less than 3 seconds, fake it. 
	) {
		// Add a rule in your inbox to look out for [!] 
	 		$subject = "[!] This is my subject";
	} else {
 		// Continue on as normal.
			$subject = "This is my subject";
	}
?>

Before Form

<?php
	session_start();
	$salt = $_SERVER['SERVER_NAME']; // or wahtever
	
	$fieldName = $_SERVER['REMOTE_ADDR'].$salt; 
	$_SESSION['captcha_field'] = $fieldName;
?>


<? // The time based captcha, needs JS to run, but most REAL people will have that ?>
	
	<input name="<?php echo md5($fieldName."_timer") ?>"  id="<?php echo md5($fieldName."_timer") ?>" type="hidden" />
	<script>
		$(document).ready(function($) {
			setTimeout(function() { 
					var dateObj = new Date();
					$('#<?php echo md5($fieldName."_timer") ?>').val( dateObj.getTime() + dateObj.getTimezoneOffset() ); 
				}, 3000); // change the 3000 into a minimum of how many seconds it would take to fill out your form.
		});
	</script>