There are read, write, and execute permissions to consider, too. Denial of service should be hard. But you can degrade gracefully under attack, as opposed to simply going belly up. Ideally, you also want to make forgery as difficult as possible - although, regrettably, SMTP itself makes forgery trivial. You want to avoid theft of service, aka spam.
General security principles Minimal permissions.
Things should have the minimum permissions they need in order to operate. Tradeoff between usability and security. As the example of permissions also shows, there's always a tradeoff between usability and security. The windows in your house make it more usable, letting light in and letting people see out. But it's easy to break a window to get in.