20 years ago I went to work for a company that used Notes internally for email. I have been using email since the ARPANET days, and have used many products, but Notes was particularly bad. But I think I understood its reasoning. Back in the 1980s, before the Windows desktop monoculture (with Mac as the official opposition), there were a lot of different desktop environments. VMS, Unix, Wang, DG, IBM PROFS, etc. So there was a push for programs to be consistent across OSs, in case the user was shfted from VMS to Solaris or something like that. But that made the programs inconsistent with their host OSs. So Notes Mail on Windows was more like a foreign application. And not a well-designed one at that.
Conceptually, however, Notes was good. Ray had apparently worked on DEC Notes, a very different and simpler text-discussion program that did what it did quite well. Lotus Notes let you design database applications in it. But many people instead just used its half-baked mail program. I used it, but did more using my unofficial Eudora client on an outside system.