Ya he encontrado el error. Al final debe ser así:
Código PHP:
<input name="email" type="text" class="<?php if(eregi("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$", $email)) {echo "inputbox-normal";} elseif (isset($_POST['email']) && empty($_POST['email'])) {echo "inputbox-rojo";} else {
echo "inputbox-rojo";
}?>" id="email" value="<?
if(isset($email)){echo "$email";}?>" size="15" maxlength="30" />