Form Validation & Security?

Hey all, I'm trying a little project to help improve my php and mysql knowledge.

I'm struggling with email validation and security.

I'm looking to check if an email is valid or not before I add the ability to add it into a database.

What I have so far:

<?php
	$errormsg ="";
	if (isset($_POST['adduser'])){

	$email = mysql_real_escape_string($_POST['email']);
	$pword = mysql_real_escape_string($_POST['pword']);
	$pword2 = mysql_real_escape_string($_POST['pword2']);

	if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
	
		if ($email == "") {
			$errormsg ="Error, You must fill in the email box.";
		}
		else if ($pword == "") {
			$errormsg ="Error, You must fill in the password box.";
		}
		else if ($pword2 == "") {
			$errormsg ="Error, You must fill in the repeat password box.";
		}
		else if ($pword != $pword2) {
			$errormsg ="Error, Your passwords don't match!";
		}
		else {
			$errormsg = "Success!";
		}
	} else {
			$errormsg = "Invalid email format, please use a valid email address.";
	
	}	
	
}
?>

This always outputs the error message "Error, You must fill in the email box" now, however without the Filter_Validate_email it outputs success when all boxes are filled in.

Anyone got any help on a ) whats going wrong and b ) any other security features I can add?

Thanks!