[SudoAlex]

Or you do nothing wrong on your server, but another user using your mail server ends up with a compromised password, or reuses the same username/password from elsewhere as their mail login. You wake up and see thousands of outgoing emails in your mail queue, all from the spammer, followed by other users who can't send outgoing emails.

You could attempt to limit the outgoing emails per account per hour, however if that's set too low then you end up with other users who can't send out emails to their "mailing lists" consisting of hundreds of contacts, instead of using a real mailing list to manage it.

The effort just isn't worth it.