I meant Outlook Express (the one bundled with the Windows) - I assumed it would be more clear from the context.
I *HATE* pseudo "ecosystems"... it's just a fancy monicker to creating incompatible walled garden, that's sole purpose is locking-in users so they can't migrate off...
Yeah, Outlook Express was never of much value (but about like Thunderbird, perhaps a small step below Thunderbird), just trying to gain on the Outlook brand, which deservers its position as the most powerful email system on the planet.
But I don't understand the criticism of the existence of ecosystems. That's not the same as a walled garden at all. A walled garden is exclusionary (hence "walled"). What is the alternative to an ecosystem for a large organization that would be better than a smooth working ecosystem? Obviously any company is going to put more effort to ensure they products work well with their own other products, and shouldn't they?
OneDrive works with everything, because it's embedded into the OS in recent versions of Windows, but other MS apps leverage this integration better. Same with SharePoint, Teams, Office, Exchange, Outlook, etc. All of those apps work well with everything, but work better with other MS products because MS puts in the effort to make that happen. Nothing stopping other companies from doing the same, but MS does it better.
Seems to me the alternative would be something like the Linux world on PCs (not Android -- Google has locked that down far more than MS has locked Windows), where everything is open source, so everything kind-of works with everything, but only for users willing to put in a huge amount of effort compared with the Windows counterparts, hence its appeal to some engineers (who enjoy the challenge of putting in that effort) but not to the mass market who lacks the skills or time to spend doing all that extra work.