How did the email client industry develop?

The history of e-mail is longer and deeper than you have given it credit for, and the examples you mention are from different eras of its development.

Most of the key developments are not in the clients as such, but in the hosting - where is the e-mail stored, and what other services is it bundled with? Alongside that are wider developments in how users connect to networks, the protocols those networks run on, and the types of user interface available.

Some key milestones:

• 1960s to 1970s: multi-user computers and early networks. Inter-user messaging in various forms is an obvious feature, and implemented many times in many different forms.
• 1980s: dial-in Bulletin Board Systems for home computers include user-to-user messaging via proprietary command-line and menu-driven interfaces. Some are local-only, but some are connected into larger exchanges such as FidoNet.
• 1982: First specification of SMTP, the Internet-based e-mail protocol we still use, with its user@host address format.
• 1984: First version of POP protocol for accessing remote mail boxes.
• 1985: What later becomes AOL launches; essentially a graphical BBS client aimed at non-technical users. Includes its own messaging services, and is not connected to the Internet.
• 1988: First version of IMAP protocol as alternative to POP. POP3 (the version still in use) standardised.
• 1991: World Wide Web announced to the public; the first popular graphical browser, NCSA Mosaic, came out in 1993, and the first version of Netscape in 1994.
• 1990s: AOL connects its users to Internet services, and heavily markets itself as "the way to get online". One of its main competitors is CompuServe, which is older and similarly started as a non-Internet BBS.
• 1990s: Meanwhile, dial-up ISPs offering direct Internet access are set up, usually offering each account one e-mail address for use with a standalone mail client. Netscape, the most popular browser of the time, included a graphical e-mail client for connecting to POP3 and IMAP servers.
• 1996: HotMail and its early competitors launch, giving people access to a mailbox over the web. This allows users to access their e-mail from any internet connection, and separates their mailbox from their ISP subscription, but ties the "client" (the web UI) to the service (unless they also offer remote access over POP3 or IMAP).
• 1996: Microsoft release the first version of Exchange, a proprietary mail server and client for businesses. A year later, a version is released that can handle standard SMTP mail.
• 1997: Microsoft buys HotMail; Yahoo! buys competing service RocketMail and relaunches it as Yahoo! Mail
• 1998: AOL offers a web-based UI for its users to view their e-mail. This was a common feature as ISPs wanted users to continue using their ISP-provided addresses, not move to a webmail-only host like HotMail.
• 2000s: Dial-up internet gives way to ADSL, massively increasing speeds for home users. A competitive ISP market makes web-based mail attractive to home users who frequently switch providers, but businesses generally own their own domain names, so alias them to ISP-provided services.
• 2004: GMail launches, with a hugely effective viral marketing campaign based on users "inviting" each other to the service. It is seen as "more modern" than established services like Hotmail, but also has a massive storage quota of 1GB rather than a few megabytes.
• 2007: GMail offers IMAP support, allowing use of a GMail address with a third-party mail client.

Through all of this timeline, standalone client programs have been developed for connecting to the various proprietary and standard protocols. Some were bundled with other products; some were the first or only choice for a particular platform; some were heavily marketed; others simply came and went unnoticed except for a few loyal users.

Oh yes. I'm old enough to have witnessed everything. First a dial up account with forum software. No internet as yet, you connected to a bulletin board. A bit later a dial up account without graphics (bloody expensive back then) and Internet. Connecting through Trumpet Winsock. Speed was, if I recall, 14.4 k. When I got my Internet account, one of the first in the country, I had to go to a university for a day to get trained in using it.

My email client back then was Pine. My browser was Lynx. I would go online, look for something quickly - surfing was expensive! - download that page and read it offline. Google wasn't created as yet. Yahoo was barely there.

Hotmail was international, AOL was American - more popular in the USA - and proprietary. Because Hotmail was international it was more popular worldwide.

How did they make money? They simply charged you for it. Yahoo tried a different approach: they allowed free searches with their search engine, but if you wanted a good listing, you had to pay for it.

That's where Google jumped in. Nobody liked the paid search results of Yahoo. First of all, you knew it was paid for. So how reliable was it? Google also had paid links, but far more discrete. A few small ones above the page, and a cheaper version on the right hand side. Good enough to get attention, and far less irritating than the Yahoo ads. That's what killed Yahoo - and very fast. Within a couple of years Yahoo dropped from the number one spot to one of the many.

What is an "email client" in the first place?

The Wikipedia version of History of email indeed glances over the popular aspects.

In Bulletin Board System we had email in almost closed circles, sometimes wider networks and gateways into the real internet mail-system. (Example FidoNet). That started at a fairly popular level during the 1980s. On Amiga, Atari, Mac and PC this was as text-based as it gets, often menu driven. But also the smaller niches were catered for like C64, other "home-computers". But also on the "bigger iron" there were things like mailx (not even menu driven) or elm (released in 1986, and even today perfectly usable 'UI').

But popular means graphical so that the dumbest user can point, click and make a mess of the place called internet.

Enter the Pegasus and Eudora in 1988/9. These programs were real clients with a GUI that popularised email to a really wider audience. Easy to use, nice to look at. If you were "connected".

That makes the difference here since AOL was really popularising "connecting to the net". The popular phrase for this "democratisation" of the internt was Eternal September of 1993. Like other middle men of the time (compuserve, eworld, etc) it offered internal mail-like messaging and a gateway to the real internet. But despite including a "client", it still was a middle man, and as middle men do, it was charging you as well. Making it cumbersome and any user depended on AOL to "get there".

Hotmail was not a "client" but a free service on the "real internet"; offered free of nominal charge ("free" in the sense as "if some offers you something for free on the net, you are the product being sold"). As real and direct connections to the net proliferated, direct access to a POP-mailbox got more convenient than going through the closed ecosystems of these walled garden providers like CompuServe or AOL. Some of these services were client based, but with a service on the WWW-web. Even that got increasingly superfluous in the eyes of casual users, requiring only a measly browser to get stuff mailed.

The advertising frenzy with AOL-discs produced to make landfills overflow in a scale several times bigger than VCS E.T. cartridges might have helped a bit. But the underlying infrastructure of the net available to home-users and adopted in businesses made the change much more attractive. Dial-up only ended when DSL came into numbers, so the change-over took from 1993–2000 in most developed areas.

For a while, many people had the impression that in order to "go online" one needed AOL, a reason deduced from watching TV or opening one's physical mailbox in front of a house (Youtube: AOL Commercial Video). But hotmail capitalised on being a viral campaign on the net itself:

Initially, each Hotmail message arrived in someone’s mailbox mentioning the words in the signature of each email: “Get your free email at Hotmail” at the bottom. By clicking on the word Hotmail, the recipient was taken to Hotmail’s home page where the free e-mail service was explained. Back in the nineties most computer users paid for Internet service which included one e-mail address. A big benefit of having a Hotmail e-mail address is the fact that subscribers can access their account from anywhere in the world, instead on their home computer. And the fact that there are no extra costs for each extra email address, Hotmail’s approach resulted in thousands of sign-ups each day.

So for Hotmail, this viral approach was extremely cost-effective and successful. While one of Hotmail’s competitor spent $20 million to attract less than half of Hotmail’s subscribers, it spent only $500,000 over a 2 year period to gain 12 million subscribers.

https://www.youtube.com/watch?v=1npzZu83AfU Really going from AOL-Client (Launched March 21, 1993) to the WebMail systems of old (first 1994: PTG Mail) and then H o t M ai l in 1996 to the GMail-Hype in 2004 we're looking at very different things. Just looking at the timeline:

A BRIEF HISTORY OF EMAIL APPS

and the volume of mails sent, expressed in changes of volume:

The History of Email

Of course, the total number of mails just keeps on growing, as ever:

The Shocking Truth about How Many Emails Are Sent