Add custom auto-archive (to in-place mailbox) policy in Microsoft 365

The default auto-archive policy for Exchange Online mailboxes moves messages older than 2 years to the in-place archive mailbox. If you want to adjust the timing, you can login to the Microsoft Purview portal and create your own retention tag and policy and apply it to specific (or all) users as required: Customize an archive and deletion policy (MRM) for mailboxes | Microsoft Learn

Note that Microsoft state: “After you apply the new retention policy to mailboxes… it can take up to 7 days in Exchange Online for the new retention settings to be applied to the mailboxes. This is because a process called the Managed Folder Assistant processes mailboxes at least once every 7 days. Instead of waiting for the Managed Folder Assistant to run, you can force this to happen by running the Start-ManagedFolderAssistant cmdlet in Exchange Online PowerShell.”

To kick it off, open PowerShell, then run the following commands:

Connect-ExchangeOnline
Start-ManagedFolderAssistant -Identity user@example.com

As an aside, you can check which policy has been applied to users with this command:

Get-Mailbox -Identity user@example.com | Format-List Identity,RetentionPolicy

[NOTE: You can omit the -Identity user@example.com parameter from the above command to list the policy applied to all users]