EDIT:
> When someone decides to run a persistent brute force attack from a botnet, eating up 100% of your CPU and you have no meaningful ways to block it.
postscreen? http://www.postfix.org/POSTSCREEN_README.html
BTW, there is soo much FUD in your comment, check http://www.postfix.org/ before claiming "someone will hack your email"
""" First of all, thank you for your interest in the Postfix project.
What is Postfix? It is Wietse Venema's mail server that started life at IBM research as an alternative to the widely-used Sendmail program. Now at Google, Wietse continues to support Postfix.
Postfix attempts to be fast, easy to administer, and secure. """