In Outlook Web App, it is possible to define Retention labels for e-mails on custom folders. Folders you cannot put such labels on are: Inbox, Draft, Sent items, Deleted items, Junk Email, Archives and Notes.
Right-click the folder and select Assign Policy from the context menu.
Such labels could be that all e-mail in the folder older than 1 month, 6 months, 1 year or 5 years should be deleted automatically.
You can also define that e-mail in a certain folder should never be deleted or that for the folder you should use policy for parent folder.