Basically, this document is telling you that the Inbox is most expensive container in a mail file. It is updated each time a user opens the mail file or clicks refresh button. The Inbox is also where new mail is delivered. Reducing the number of documents in the Inbox reduces CPU utilization as well as the space required to update and maintain the view index.
Domino 8 now allows administrators to schedule a time to clean up the inbox of some or all mail files. Keep in mind that, other than the ($Inbox) view index, that this will have a negligible effect (if any) on the storage used by the mail file. The documents are only removed from the Inbox and will remain in All Documents or any other folders in which messages may have been copied. In a phased-in approach, Inbox Maintenance can be a useful (blackmail) tool to get users to begin using folders and manually cleaning up the Inbox (I think we have all seen the users who feel they need to keep all of their mail in their Inbox, right?). For instance, you could choose to notify users that in 30 days all messages over 180 days will be removed from the Inbox of all mail files.
There is a 2 step process to enable this:
1. You must edit the Server Document. Go to the Server Tasks tab and then to the Administration Process tab. At the bottom right, you will see this:
As you can already see, the Inbox Maintenance is a function carried out by AdminP. In the Server Document you can control the day(s) and time this will run. You may also choose to run this only on specific users' mail files or on those files maintained through policies. Along with the schedule specified in the server document, you can also run Inbox Maintenance by issuing the TELL ADMINP PROCESS MB command. If you choose to run this on selected mail files, you are able to choose how many days to retain and whether to remove unread messages.
2. After modifying the Server Document, you may want to use policies to control the subset of mail files to maintain. You will need to implement this through a Mail Settings document in a Policy. In the Mail Settings document, go to the Mail tab and then the Basics tab. The Inbox Maintenance section is at the bottom: