Hi @Kurt Stichelmans
Welcome to our forum!
It sounds like you might need to check a few things to make sure everything is configured correctly and to understand why some items aren't moving to the archive mailbox:
Outlook Rules: Verify if there are any Outlook rules that might be moving items to the archive folder.
Mailbox Size: Check if the mailbox size is less than 10 MB. The retention policy doesn't automatically run for mailboxes smaller than 10 MB. If it is, you'll need to manually run the Managed Folder Assistant.
Retention Policy Configuration: Verify that the retention policy is correctly configured to move items older than 1 year to the archive mailbox.
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