How can I mount a data file (pst) in new outlook. In the classic version this is doable through the File menu which is not available in new outlook.

Sorry, but the "new" Outlook is ABSOLUTE GARBAGE!!!

You either don't understand the question or are unable to provide correct ansver - THER IS NO FILE tab in the NEW (much worse than the OLD Outlook

Adding an update on this thread from my comment many months ago.

Wanted to clarify for folks that when there is a reference to the "old" Outlook, this doesn't have to be a significantly older major version - perhaps the more applicable term would be "Classic Outlook".

For those who are M365 users, this is what I've found userful for myself, and also in deploying new endpoints to our team members:

1. Download & install the desktop applications of Outlook on the (new) device.
2. If you're a M365 subscriber, you should be able to do this by going to "office.com", make sure you're logged in, and from there you should see a button on the right side of the page below the "Microsoft 365" banner that says "Install and more". From there, you can download the apps - once downloaded, install. (Screenshot below)
3. After installed, open Outlook. As of last week when I deployed four new endpoints, it's going to default to the "new" Outlook. But from within the application, you can go to "Help" in the top bar, and from there you should see an option to click to switch to "Classic Outlook".
4. To make things clear for my endpoint users, after it opens the "Classic" Outlook, I immediately pin it to the Taskbar, (and unpin the "New Outlook" if it was there), that way the users are clicking to open THAT version every time.
5. Within the Classic version you can import your classic .pst files, and create new ones as needed.

Hope this helps folks! (And I hope this work-around remains viable without any further forced application updates from MS).

You might find the return to old Outlook switch on the Help menu now...

Concur with everyone's feedback on the 'new' outlook.

Fortunately I was able to turn off the new Outlook and it reverted to the old version with my .pst file/s were intact and re-enabled.

I'm truly at a loss as to why so many features were removed from this new and improved (sic) version! E.g. I was amazed that conditional formatting was no longer available when this was a fundamental feature in the rules section - I had it configured to help me quickly identify urgent / critical emails by colour. (what I had to read/action urgently vs. general info).

What a cluster....