阻止来自“告诉朋友”表单的垃圾邮件

You have to have a form on your website for people to send an email to a friend if they found something interesting. You can force people to be logged in (which is not a good option in my case). You can make time delay (this is not really urgent email, so it can wait for 5 minutes). Do you have this problem? How would you solve it?You have to have a form on your website for peo