Hi,
According to my research, I am afraid there is no built-in method for this. And as far as I know, Outlook should remember which folders were expanded or collapsed when you closed Outlook and should also start with this folder state again, so please
always close your Outlook client with the folders expanded as a workaround.
For your reference: Starting Outlook with all folders collapsed/expanded.(Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the
accuracy of this information.)
Hope this can be helpful.
Regards,
Aidan Wang
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