How do I prevent Outlook 365 on the Mac from launching the New Version

I am in exactly the same situation, I have been forced three times to convert to the "improved" version which is, in reality, a nightmare. I instantly see random emails strewn everywhere with thousands of disorganized emails suddenly in my Inbox. My Contacts disappear along with my Calendar, and worse still, all my stored email Folders, which contain business records going back to at least 2006, treasured notes from friends and relatives (some now deceased; Bill Gates how would you like to lose yours?), important estate planning documents,all are gone in an instant. So far, fortunately, I have been able to revert to the Legacy version of Outlook, and all is just as before. But I'm not terribly skilled on computers, I'm not getting any younger (mid-70s now), and I very much fear that one of these days I will not be able to revert to the old version once the dreaded New Outlook prompt appears. In fact, although I think I know how to open Terminal and try the fix that RandallPacker above has suggested, frankly it scares the you-know-what out of me to start fiddling under the hood of my new Mac M1. MICROSOFT YOU SHOULD BE ASHAMED OF YOURSELVES FOR PUTTING YOUR CUSTOMERS IN THE ROLE OF VICTIMS!

If anyone knows of an easy-to-implement alternative email program that this senior citizen could switch to with some support, I'm all ears. Ironic that I am typing this note on a day that MSFT stock reached an (undeserved) all-time high.

Thanks for listening (?). Kind regards, George

Thank you, this is working!!!

In reality, this instance is still occurring today. The Outlook team has failed to take any action to assist their users. Even the new Outlook for Windows is a disaster.

I'm not sure if the UI/UX Team uses Outlook for routine email work. Why can they come up with such a design while still removing the vital features?

That seemed to work! Thank-you!!!!

Even after entering the terminal command, Mac Outlook is forcing me to be offline or "upgrade" to Office 365.

This seems illegal to me. I paid for Mac Office 2019. Now Microsoft will not let me use what I paid for.

This just happened to one of my clients, they hate the new version and the forced "upgrade" didn't migrate a couple of accounts so I had the manually add them back. But then it would only sync the last 2 days worth of inbox from the server. Whilst I was trying to fix this mess I stumbled across this thread - before opening up terminal I followed another link in a comment by @Mr. Richard P (back on page 2), and linked off that page I found this page

https://support.microsoft.com/en-gb/office/the-new-outlook-for-mac-6283be54-e74d-434e-babb-b70cefc77439

There is a way to revert back to the legacy version, there is a link in the Help menu.

Clicking, opened a modal asking why I want to revert - so I told them, then it updated the database, quit and reopened in the legacy version.

Who knows how long this option will stick.