As a customer who has leased a dedicated box, you have complete control on the limits. You can set no limits for outgoing emails. However, just imagine you have hosted a website with a custom email script in the server. Now that script has some security loophole and is exploited by some spammer to send out spam emails. Imagine, he is sending out over a million spam emails from the server. What will happen in this case?
1. The cpu and memory resources will be hogged.
2. There will be a huge I/O.
3. There will be lots of emails in the queue waiting to be delivered which means the legitimate emails will have to wait.
4. Huge load on the server will result in the websites slowing down.
5. The IP will get blacklisted in RBLs.
Of above points, the IP blacklisting will happen even if there is a single spam email sent. However, putting a limit on the number of emails that can be sent, you are actually preventing lots of issues. I remember one of my customers had made a mistake and his email queue was filled up with 7.5 million emails. We had to reboot the box and upon reboot, we had to stop the email service till we deleted every single email from the spool. It was only after the emails were deleted, we could access the server and put limit on the number of outgoing emails. So, even if you have control on the dedicated server resources, the resource allocation should be done smartly.