Control Theory is applied mainly to electronic systems, but it's equally applicable to all systems everywhere, with no exception. That includes networking, and it even governs human systems.
It's a truism in Control Theory that a system without negative feedback is a system that is out of control. All non-trivial systems without negative feedback head towards an uncontrolled state on the slightest perturbation of initial conditions.
Email is one such system. It was designed without negative feedback back in the early days of the academic Internet before malicious actors appeared on the scene. Because there is no "cost" associated with sending an email, the system went out of control --- the primary effect of that is spam. (This "cost" has nothing to do with money.)
In Control Theory terms, "cost" is any control metric that tracks an undesired effect and reduces that effect when applied to its cause. One of the most universal undesired effects is resource consumption, and that's directly applicable to the email problem because many kinds of resources are used up by spam when it arrives at MTAs and at end-user mailboxes --- examples are CPU time, storage space, network bandwidth, end-user time, and many other things. They're all resources, and spam is the direct result of the spammer feeling no "cost" when he consumes other people's resources. There is no negative feedback being applied to his posting of spam.
"Cost" in the control theoretical sense could be many things when applied to email, for example a slowdown in the spammer's ability to post his next email proportional to the rate of sending and to the number of recipients. There are dozens of possible ways to make a spammer feel a "cost" as negative feedback for his actions, many of them leaving normal mail users entirely undisturbed by the negative feedback. Unfortunately email has none of these control methods available, and it probably never will because it's too late in the day.
One day however, a new asynchronous communication protocol will be designed to replace SMTP. It must be designed with a mechanism for negative feedback integral to the protocol and non-optional, or else the spam problem will appear again, sure as night follows day.
Note that we have many other systems out of control in computer networking, it's not just email. For example, there is no negative feedback applied to rampant abuse of user-side scripting by web pages. Web developers feel no cost regardless of how much end-user CPU, storage, or network bandwidth they employ, and since there is no negative feedback applied to their over-use, browsers typically have their CPUs pegged at 100% and the Web has turned to molasses. As techies we try to control the Web excesses with NoScript (for example) just as we try to control spam with SpamAssassin, but these are just fighting symptoms. You can't cure a disease by fighting symptoms.
This is a universal truth. No negative feedback spells trouble ahead.