To apply the Retention Policy to all mailboxes:
Get-Mailbox | Set-Mailbox -RetentionPolicy “name of policy”
To verify which mailboxes the policy was applied to:
Get-Mailbox -Resultsize Unlimited | select Alias, RetentionPolicy
To run the policy manually:
Start-ManagedFolderAsssistant or Start-ManagedFolderAssistant -identity “user”
To apply policy on non-default folders like Inbox, Sent and Deleted items:
Create a new Retention Policy Tag for “All other folders in the mailbox”. Just a word of caution, this policy will also be applied to NOTES and CALENDAR items. If you do not wish to include these items, you can create a separate policy for these items and set to ‘Disable this tag (items are never expired, moved or deleted).