Hello,
My client used some SEO tool to crawl the website and then the bots used the Email to Friend feature to send numerous mails. Every minute few emails were sent.
I solved the issue by Enabling the reCaptcha.
Below link has some screenshots which might be helpful to someone:
http://imgur.com/a/pvOe0
Regards.
Pramil Gawande
+91 77 55 90 2019