Every time an email is sent, it’s expected to be handled to its recipient, no matter what their service provider is. But in the first days, it wasn’t so simple: early commercial email services (CompuServe, Prodigy, Delphi, …) featured proprietary email services with no concept of an universal e‑mail address, which in turn created technical barriers that originated the commercial practice of settling charges for email delivery between service providers. In other words, there were interconnection agreements which detailed the delivery charges between providers, bounding the two parties to periodically settle their accounting differences, much like in the other telecommunication networks (telex, fax, teletex, SMS and phone termination fees).

But the number of said required agreements grew exponentially as the number of service providers expanded, and so did the technical difficulties to integrate their different email services. X.400 was born to solve these issues, implicitly providing support to keep settlement scores between carriers and their multi-interface delivery technology (vg. the preferredDeliveryMethod attribute). In the end, X.400 didn’t really take off and was substituted in 1990 by the much simpler X.500 protocol: but not due to its tremendous complexity, but rather due to the decisive move of service providers to stop settling accounts between them so they could just use X.500 to interconnect their directory services.

As usual, it’s almost never about technology, which can be better thought as the child of necessity and will. The hassle of reaching agreements was getting so high with the growing number of service providers that their diminishing return stopped justifying the related bargaining costs, which in turn were precluding the emergence of the essential network effects from the growing number of email users (as per Metcalfe-Beckstrom-Reed’s Laws): that is, they were the real limiting factor blocking the growth of the early Internet. Nowadays, the only trace of these agreements survives in transit traffic agreements, in turn solved by peering agreements.

To sum up, notice the circular paradox that the history of email established, a curious tale of unintended consequences: free email begot spam, and spam beget the obvious solution to start charging for email to put an end on it. Whether the trade-off was correctly solved depends on whom you talk to.