Ever since I hired a Chinese developer on oDesk and gave him access to my sites cpanel I started to get these messages from my VPS provider that my server is sending out millions of emails per hour. So their system was suspending my server and killing all of my clients sites. This was happening every other day, for the past several months. I just kept bitching at my VPS provider that I am not sending them. Then finally I decided, maybe I should look into this and fix it myself.
I finally took care of this issue with 2 simple plugins.
andBotnet Attack Blocker Temporarily block all admin logins after multiple failed attempts - helps to prevent brute force botnet attacks from multiple IP addresses.
You can easily find them via the Admin plugin panel in Wordpress.HC Custom WP-Admin URL
Small and simple plugin that allows you to change url of wp-admin
Ever since I installed these plugins, I have had no attacks whatsoever.
Hope this helps one or two people.