Microsoft is currently pushing users to "beta" test the new Outlook in the same way as the new Teams client.
The new Teams desktop client for Windows is a big step forward, means you can finally be active in different tenants. Previously you had to always switch tenant to receive notifications from other tenants.
Now the new Temas client allows you to receive notifications from all tenants which you are logged in. Hurrah, MS did finaly understand how modern working is, you are often not only in one company, but you have different teams and client connections. (Of course all major competitors like slack did have this feature from the start, many many years ago)
So now the new Outlook client suggests that you will also receive better user experience compared to the "old" one.
Before switching to the new desktop client you have to know of a few (breaking) changes
1. You old addins won't work any longer, you have to look if a web version of these addins exist and install (and perhaps purchase) these. If they don't exist, your out of luck. But on the plus side, if such a plugin exists, you will also have it available in the web frontend
2. You will finaly be abke to drop the old pst files, together with all the problems comming along with them. On the backside, the search (or better the find) capability in outlook across accounts is not existent. Also moving emails/calendar entries between accounts isn't fun with the new interface and you can't automate these any longer
If you can live with these, then you can consider using the new client, but stop, there are two more things, which microsoft did not tell us initially:
1. All acount data of other smtp/imap/pop3 accounts, together with all the emails etc. are sent to the microsoft servers. And yes, it's not a bug, it's a feature, to allow "better user experience" across systems. And there is no way to opt out of this account stealing, togerther with all your emails from foreign accounts
2. You can't use the new outlook desktop client to access exchange mailboxes, which have only a exchange online 1 or 2 license. If you wish to use it, then you have to upgrade the exchange online 1 or 2 license to a Microsoft365 licence which includes the desktop apps. So instead of paying 3.60 CHF/Month per mailbox (Arround 4$) you have to pay microsoft 11.20 CHF/Month for the Business Standard plan.
The last point is especially stange, but microsoft has confirmed this behaviour as the expected way of working (Read get more revenues). When you had added exchange online mailboxes to the "old" Outlook, then the automatic migration to the new outlook takes them over with no hick up and they work wonderfully in the new Outlook. Be beware, if you remove them, there is no way to re add them back, MS does a license check and denies you this.
Currently I have two employments with fully licensed Microsoft E3 enterprise tenants, and also 3 more tenants where I have full Microsoft education licenses. And even with these 5 Office 365 licenses together, I'm not allowed to open a simple Exchange Online mailbox...
If you continue down this road, then microsoft might publish new Excel and Word versions, which won't allow you to open/edit documents, if they are not stored on some kind of licences microsoft storage, just plain silly.
So I started using BlueMail as my mail client for the Exchange stuff and the good old Thuderbird for all non-microsoft mailboxes.