How can I retrieve migrated On My Computer data when returning to the legacy version of Outlook 365?

Question

I have a lot of old mail, adding up to nearly 100 GB of data. That took days to get migrated from the legacy version of Outlook 365 to New Outlook 365, when the Beta-version that made this possible was just available (2022). It’s all there. However, searching it, takes much more time than it used to. It looks as if all messages first need to get uploaded to the server, before they can be searched, which is very different from the search performance of the legacy version. This is why I would like to switch back to legacy. Here comes the problem. When I switch back from New Outlook to Legacy Outlook, I see the On My Computer folders, but they are empty! When I inspect ~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Main Profile, I don’t see anything that looks like an SQL database. Instead, there is a folder called Omc that has a lot of subfolders that seem to contain all the old messages (in the form of separate .eml files). Could this be the root of the problem? How do I fix this?

Answer 1

The new Outlook displays emails cached on the Microsoft cloud.
Are existing emails backed up in PST files?
If you install the classic Outlook (Microsoft 365), you will be able to view the PST file.

<Japanese>
新しいOutlookでは、Microsoftのクラウド側でキャッシュしたメールが表示されます。
既存のメールはPSTファイルでバックアップしているのでしょうか？
従来のOutlook(Microsoft365)をインストールすれば、PSTファイルを参照可能となります。

Answer 2

New Outlook should be able to open PSTs now as long as classic Outlook is installed. In new Outlook, click Settings>General>Outlook Data Files