Before you turn on the “Auto-Expanding” feature ensure you have enabled the archiving option for the user.
Get-Mailbox -Filter {ArchiveStatus -Eq "None" -AND RecipientTypeDetails -eq "UserMailbox"} | Enable-Mailbox -Archive
How to enable Auto-Expanding for your organization?
Run the following command on PowerShell:
Set-OrganizationConfig -AutoExpandingArchive
If you would like to enable this feature per a mailbox you can use the following command:
Enable-Mailbox <user mailbox> -AutoExpandingArchive
Maybe have you already enabled it? Check it out:
Get-OrganizationConfig | FL AutoExpandingArchiveEnabled
Or per a mailbox:
Get-Mailbox <user mailbox> | FL AutoExpandingArchiveEnabled