I wonder what they used instead of outlook
Before Outlook there were many, many options, which is kind of the point. You might be intrigued by something like All-In-1 https://en.wikipedia.org/wiki/ALL-IN-1
My own first email client was VMS Mail at uni in 1988, then pine when I started to use SunOS and Solaris every day for work. Lotus Notes was very big for a while too before the web killed it off.
I think the architecture of Notes/Domino was technically very interesting - a rapid application development environment incorporating a replicated document-oriented database, cross-platform GUI forms designer, and scripting language.
And then that environment was used to build an email and calendaring application. Some customers bought it just for email and calendar, and ignored its potential as a platform for custom applications. Others used its application development features heavily.
But I think part of its decline was that its potential as an application development environment/platform never received enough emphasis from IBM. IBM bought it for the email and calendaring - their mainframe-based groupware line (OfficeVision, PROFS, DISOSS, SNADS, etc) was really showing its age, and buying Lotus was their answer to that business problem. And that’s how they positioned it in the market, and that became the focus of their R&D investment.
I remember people used to complain about how the Notes email UI was confusing - due to its cross-platform heritage, it didn’t use the same keyboard shortcuts as Microsoft apps, for example. (Something I believe they improved in newer versions.) Yet underneath that email client lay something powerful that its competitors (primarily Exchange and GroupWise) completely lacked
I wonder what might have been, if IBM had positioned it more heavily as a platform for applications rather than just email+calendar - or if it had ended up with someone other than IBM? IBM didn’t really need an application platform because they already owned plenty (WebSphere, CICS, IMS, TPF, AS/400, VisualAge, Informix-4GL, Rational, SAA ADCycle, Cross System Product, UniData/UniVerse, EGL, PowerHouse 4GL, etc). Maybe it would have gone better with a company for whom it was their sole or primary application platform instead of just one among many?
I’ve heard some suggestions that now HCL has bought it, they have a renewed interest in using it as an application platform compared to what IBM had. Even if that’s true, probably too late to make much of a difference-there are so many other options nowadays, arguably better.