Hi @Mountain ,
Welcome to our forum!
I notice that you added the Exchange online tag under your post, are you using an Exchange online account? If so, it's suggested that you could try to configure your account as an Exchange account in Outlook desktop client, so that emails will be kept on server.
According to your description, i notice that you have enabled the "Leave a copy of messaged on the server" and set "Remove from server after n days", right? if so, please confirm if this account is also configured on other Outlook clients and third-party devices. In all Outlook clients configured this account, the two options need to be enabled.
Now my old emails have permanently disappeared
In order to further confirm if the emails have been permanently deleted, please log in to your web mail and check if the emails are there.
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