What I did:
Open ‘Apps and Features’ with Win + I ---> Apps ---> Apps and Features
Find the pesky Outlook in this list ---> uninstall it! After OS updates check if it has been downloaded again, after updates uninstall it again.
You will also find yer beloved ‘Mail and Calendar’ in that list. Make sure it's there, if not, they've removed it (probably going to happen after Dec 31, 2024)
For this case, which may or even most likely will be happening after Dec 31st 2024, download the 'Mail and Calendar' bundle as a precaution while you still can, so you can reinstall it later. I have a version called ‘W10 Mail & Calender 16005-14326-21904-0.appxbundle’. Maybe it's still available in their Store app, otherwise get it from a site of yer trust. make sure it's the bundle, the software package, not just an msi-downloader! This wouldn't help you later, as they gonna remove whatever this downloader was intended to download.
Finally, go to that Store (the app!), if you're not already there, click on yer account avatar ---> settings ---> disable automatic ‘App Updates’! Always a good idea to keep more control of what is going on on yer machine.
For me, this forced re-re-re-re-directing to Outlook is history. They don't have anything Outlook-esque installed on my PC anymore that they could send me to in order to force me to use their latest **** like I'm their trained lab monkey.
But they still keep pestering me with a pop-up in the top right corner of Mail and Calendar every time I open it, threatening to download Outlook the next time I open Mail. But they don't. For now. I have to close that annoying popup every time with my usual ‘Eff off!’ (I'm not that pc when I'm alone with myself, as you can imagine) or ‘Shut it! On yer bike, Mike!'.
I hope this workaround is sufficient for ye to be able to keep using Mail, but you should also keep in mind that their update bots are not human and never tire of molesting users until the last one has folded and moved to the new Outlook, which incidentally looks a lot like the former Windows Live Mail, which also suddenly stopped working in W10, and which I've missed dearly ever since and hated this new 'Mail and Calendar' muck. Now I want to stick to Mail instead. Conclusion:
What I – and most other users, for that matter – hate the most is obviously not necessarily any ominous new app, but these people who keep meddling with our customs and therefore usual workflow by constantly trying to teach an ol' dog new tricks! That makes those people my enemies, even if their product was good, better than what I'm using. I don't give flying rat's patootie! My wonts come first, always. Who doesn't respect them, doesn't respect me! They have to ask me, not I have to ask them, or they can shove their whole OS up where the sun don't shine, and I certainly don't mean London! There are others, I don't need them. So, they better tread lightly, or else! We're living in the world of 'or else'. God gave us baseball bats against our enemies. Ok, he gave us trees, but we knew what he meant.