"All Inboxes" setting for the new Outlook that is part of Windows 11 (not Office)

this is a pile of

Thunderbird is so bloated and slow. eMClient is smooth, fast and lightyears better

I agree with you that Thunderbird is a little bit slow, but I also got some problem with icloud and my work mail with it (I could not make the signature work).

Still I think eMClient is not a good suggestion for two reason:

1. It is not open source... and since we got this problem with microsoft it is not a good idea to get another closed source software. With an open source alternative if they change something stupid like in this case any user with a developer background can try to make a working fork. That's not possible with a closed source software.
2. eMClient that you suggest is limited to only 2 mail in the free version. If you want a free closed source software I think bluemail is an alternative to try.

After some weeks of trying everything I think I found a good alternative for me: MailSpring, it is open source, it work with iCloud mail and my work mail got the signature right as a custom html (possible in thunderbird too but when I tried it with a logo it was blocked by the email firewall of my work mail).

The only strange limitation that I found is that MailSpring do not allow to delete single email from the trash, only empty the trash.

Of course it got "All inboxes" feature. And it is fast and light with a modern interface that I like (thunderbird was not that great on the UI too...).

It got a paid version too but that's only for really not necessary stuff like send later (I found it in the free version too, not sure why... maybe in the paid version they let you decide the exact date/time), snooze messages, automatic translation and follow-up reminders... things that I actually never use or that you can use on other applications... the best thing is that if you don't want those you can skip the account creation and just use the email client as a email client... it still got everything needed like rules, signature, folders, models (it should be premium but I found it there so maybe the premium version got more), notifications and of course unified mail (not just inbox, but also any important folder like sent, trash, spam etc...).

Other open source alternatives that I did not try are:

Claws Mail and Sylpheed...

But both got a UI from last century that I don't really like.

Most of the other alternative are Linux esclusive... and I'm actually thinking about switching now that with Steam Deck the game part of things is a lot better compared to the past.

I'm still using outlook on mobile where it got unified mail... not sure why they removed it on PC, but I think most people will switch to an alternative as soon as possible.

PS. My experience with MailSpring is limited to: GMail (both personal and my old university mail that still work after more than 5 years, it will connect with oauth 2.0, really easy), Yahoo mail (it will require a password app, not required in outlook), iCloud (it will require a password app, it required it in outlook too), libero (an italian provider... it's the only one where I had to use a generic imap mail, it will require a password app but it will not ask for the configuration like it did in outlook), Outlook, my work mail (that's a office 365, not sure of the difference between outlook and office 365 mails but they both work). And it is working with my 8 mail accounts configuration without any issue on the speed side.

Thunderbird is so bloated and slow. eMClient is smooth, fast and lightyears better

Don't worry about it Microsoft. I too have gone over to Thunderbird, and am perfectly happy with it.

EmClient has the unified box and runs smoothly and quick. Microsoft has been lying about including the function for a long while and apparently they have some assumption they can force us to use an inferior app or pay to upgrade for a feature that has always been in previous versions. So, liars and stupid, surprise.