I'm assuming you mean email and will answer accordingly.

To start with it's important to understand the nature and history of the early internet. Going back to the very start you have the Arpanet project, which developed standards and protocols for creating wide area packet switched networks (most especially the implementation of the tcp/ip protocol). This made it possible for servers and systems on "local area networks" to communicate with one another across large distances even with very disparate operating systems (such as VAX/VMS vs. Unix). However, the Arpanet was not the internet, it was just the seed kernel of it, it spawned other WAN projects such as NSFNet, and MILNET, and others. These networks were all interconnected but mostly in a sort of daisy-chain fashion. If you've ever been in an old house without hallways where it's just rooms connected together that's sort of the way the collection of WANs were up through the '70s. This transitioned somewhat incrementally toward a system with a proper higher speed backbone and dedicated routing infrastructure through the '80s and '90s. Toward an "inter-network" or internet system where the forest finally became apparent as something other than a collection of individual trees.

In those early years up through the early '90s the vast majority of users of the internet were in research, academic institutions (students and faculty at colleges and universities), government roles, or in certain technical industries (especially if they worked with the government). They were professionals or academics. As such the etiquette of internet use was oriented around that membership and those relationships. In that era nobody on the internet was anonymous and "bad behavior" or deviation from the standards of etiquette which had developed through the '80s was easy to correct because internet users had a lot on the line. They were representing the businesses they worked for or they were responsible for their behavior as students. For many years during the growth of the early internet through the '80s there was a phenomenon where a whole new group of freshmen would begin at colleges across the US every September (which was at the time the overwhelming center of the internet, vastly more so than today). These freshmen would be unfamiliar with internet etiquette and of the boundaries of what was considered acceptable vs. rude behavior so they would often cause grief to the existing internet userbase as they visited various shared spaces (such as usenet, email mailing lists, shared servers, and so on) and simply made use of things like email, talk (real-time chat on the same server), etc. They would become educated by others on these points of etiquette and their behavior would typically settle down very quickly and conform to the existing norms of polite internet use. These students understood the importance of making a good impression on all of these other academics in their fields and others, some of whom they might work with or study under, and they also understood that if they were to engage in reckless bad behavior of harassing others then it could lead very rapidly to bad results for their economic and academic futures, even up to being expelled from their institutions. The folks using the internet in those days often had very powerful positions in business, in the military, and at academic institutions. There's only so much horsing around that one does when you are wearing a nametag that says "Hi, I'm Joe, I'm a student at XYZ University" and the "audience" includes a bunch of professors at ivy league universities. If you're misbehaving in a manner that is far outside of the bounds of civilized behavior then it takes just the slightest effort for one of those professors to contact the school you go to and for you to experience some pretty severe consequences as a result. And because of those incentives promoting amiable interactions and discouraging bad behavior the internet was for the most part a pretty collegial place to be in those days, even the unmoderated shared spaces.

For similar reasons at a software level the internet and its components were often hilariously open and naively insecure back then as well. Again, the assumption is that this is a collegial atmosphere where folks are acting professionally so everything started out wide open. I'll provide some examples just to provide some flavor of the way things were back then (keep in mind things were this way all the way up through the mid '90s).

Perhaps the basic building block of the internet back then, certainly at colleges, was a large shared mainframe system running Unix. To use it you would connect to it via a terminal or, more commonly, a software based terminal emulator. This would provide a command line interface that would allow you to run commands on the system and share its use along with anywhere from a handful of other simultaneous users up to thousands. Through this terminal interface you could read your email, read usenet newsgroups (somewhat similar to reddit or BBSes), make use of the system as a computer (e.g. write and run software and so on), or use other utilities. There were lots of different utilities beyond simply email for interacting with other users. For example, you could run a command called "who" to see a list of other users who were currently actively logged on to the server. You could use the command "finger" to find out information on another user, such as their name, status (online), phone number, office number or address, and even dump info from special files they created in their home directory (.plan, .project, .forward, etc.) telling you things such as what they were working on currently. You could use the command "talk" to initiate a two-way real-time (instant messaging) chat between your terminal and another user's. You could even use the command "write" to send text directly to another user's terminal.

Then you have the implementation of email via SMTP (the Simple Mail Transfer Protocol). The way SMTP worked was very primitive. To send an email message to someone on a server your email client would simply connect to that server (with no encryption or authentication) and execute a series of commands to construct the email message line by line (including the originator and the recipient as well as all the headers in the message and the message itself) then a signal of the end of the message (which is actually just a solitary period on its own line) followed by quitting the session. The server then takes the message assembled via those commands and sends it to the inbox of the recipient. In many cases if a message for a user on another server is created during a connection instead the server will store that message and then connect to the destination server and send the message along, helpfully acting as a relay.

These norms seemed to function just fine as the internet grew by orders of magnitude through the '70s, '80s, and early '90s but as we know they wouldn't last as the internet grew explosively and became a part of everyday modern life around the mid '90s. Throughout the '80s there were many people who weren't part of the traditional footprint of professional and academic internet users but still had computers at home. These hobbyists, enthusiasts, and others built their own parallel shared spaces such as bulletin board systems (BBSes). And with the rise of families buying personal computers throughout the '80s and '90s commercial companies began spinning up their own offerings of similar services (such as Prodigy, CompuServe, and America Online (aka AOL)). These services often provided access to email, news, software downloads, etc.

Around 1993 several major commercial ISPs, most notably AOL, added access to usenet newsgroups to their list of services. This created a never ending flood of "newbies" and folks who couldn't be cajoled into abiding by the existing strictures of internet etiquette because not only were they not necessarily professionals or academics but they could also achieve a level of anonymity that previously had existed only in areas like BBSes. This event has since become known as the "eternal September" and represents an inflection point in the transformation of the internet from one founded in academia to the internet we're more familiar with today.

At around this same time another titanic shift happened. The tiny little project that had been cooked up at CERN laboratories in the early '90s known as the "World Wide Web" experienced a major transition as the web browser Mosaic was released to the public. Other graphical web browsers had existed earlier, but these typically ran on systems academics would use (e.g. Unix or VMS operating systems) and were the result of pretty small scale development projects. Mosaic was much more polished, aimed at average computer users of all types, and was ported to popular operating systems home users were running including Macintosh, Amiga OS, and Windows. The mid '90s saw an exponential explosion of home users connecting to, making use of, and adding to the internet in every corner and this was rapidly followed by the commercialization of the internet with the first companies tentatively finding out how to use the internet for business and then in the late '90s a rapid gold rush of businesses chasing the promise of "e-commerce".

(contd...)