GNU Emacs is probably the easiest way to capture the spirit of working on the Lisp machines. Interlisp is an ongoing project that takes you even further down that road.
This is one of those articles that demonstrates why email should be distributed. Letting Google and Microsoft run email for the planet is just asking for problems. There are some technical demands to running email services, but they are still in reach of the technically inclined individual or organization. If for no other reason, it would help keep the big mail service providers honest.
At work, my employer is still running Microsoft products for the desktop environment. At home, assuming I'm using Microsoft products at all (rare), it's from inside the Chromium web browser on Linux or BSD.
I think it depends on your needs. Working corporate environments with 1000+ hosts, LTS operating systems are big help. On the other hand, for smaller cases, call it a work group or smaller, I think OpenBSD provides a base system that doesn't typically make drastic changes, along with a ports collection that does a pretty good job of keeping up with the third party applications. It's a good balance. I've recently seen some "Immutable" Linux distributions that are basically spins of upstream distributions. They leave the inherited distribution mostly alone and load the extras using Flatpak or the like. Sounds similar to BSD ports in a way.
I read this with some interest because I've self hosted personal email since about 2001, with experience in email management going back to 1995. The challenges presented are certainly true enough, but I still think self hosting is still worth it. I'd rather not share the management of private keys with DNS (DNSSEC) and email service providers (STARTTLS, SMTPS, DKIM, POP3S, IMAPS). I'd like to keep them, well, private. It also provides the opportunity to see the mail queue, and mail server logs. I find that helpful.
I'd call it more than a shell. It's more like a text oriented Lisp (Emacs Lisp to be specific) runtime environment with built in editing, interpretation, compilation, and debugging tools. It's also been a popular home for numerous applications that have found lives of their own, far beyond editing text.
Very recently, I tried using a combination of nail(1), and calendar(1) on OpenBSD as an email and calendaring combination for home use. "nail" is provided by the OpenBSD s-nail package and provides an extended POSIX mailx style environment. calendar(1) is part of the base OpenBSD distribution.
It worked far better than expected. The only thing missed was age calculations for birthdays. I never figured that out. Using the Emacs Diary provided that sort of thing though.
I've since moved to KDE Kontact for mail/calendaring. It works more smoothly if you do a lot of calendar sharing with others, but I could see going back to nail(1)/calendar(1) if Kontact disappoints.
I wanted the TI-99/4 that was marketed at the beginning of the 80s. As it turned out, I didn't own a computer until buying a Tandy PC-6 pocket computer in the mid-eighties.
Fast forward to buying the later TI-99/4A in the beige case in 2023. Booting it up into the Basic prompt gave me a nice flashback to learning Basic in High School on the Radio Shack TRS-80 Model I.
I've also bought its cousin the TI-74 BASICALC. Given hindsight, the TI-74 is my favorite TI-99 for retro computing, even though the similarity is limited to a subset of the TI Basic from the TI-99 family. The TI-74s are rugged and available. It's very useful as a desk calculator.
I'm familiar with a site that once had around 30 Vaxen for scientific computing in the late 1980s. This included Vax 11/780s, 11/750s, uVax I's, uVax II's and eventually a Vax 8800 used to feed a Cray Y-MP. All long gone. Linux has moved in and taken over.
I run email for myself with OpenBSD. The only spam mitigation is spamd(8) in blacklist mode, using nixspam. DKIM isn't checked for inbound mail. Outbound email is signed using the opensmtpd-filter-dkimsign package. There's some spam that makes it through, but not enough to take additional measures.
I also skipped using IMAP or POP3. There's a mail server with global IP addresses, that forwards inbound mail to my local workstation over WireGuard. My email clients read mail directly from /var/mail. Remote email access is via ssh terminal sessions. Not for everyone, but that's what I do.
I think running daemon mode has significant advantages. You can have frames running on multiple virtual desktops for different uses. You can also step away from a buffer, and return to it, maybe from some other location on a different computer.
One thing about Emacs, is it's not really just an editor anymore. Comparing it to other editors kind of misses the mark. It's more like an integrated Lisp development and runtime environment. It reminds me of Smalltalk environments, say Squeak or Pharo, albeit in a very text oriented way.
The world could probably make room for an integrated Lisp development environment that makes GUI programming more of a first class citizen. Maybe something like Medley Interlisp?