Some new Notes users, when they approach the client for the first time, are surprised that it's not possible to run a mail rule over previous,old messages.
You can write a new rule but it will be applied to new messages only...
In my opinion that feature should be standard in Notes, it would be handy.
Just yesterday I downloaded Mail Rule Utilities, a plugin from Bob Balfe, that will install a button on the Lotus Notes toolbar and allows the users to select and run Notes mail rules (defined in the users mailbox) over a folder (usually Inbox). Great tool!
Moreover you can read Bob's article about his developing of the mail plug-in.
By the way, I installed the plugin on my Notes 8.5.2 beta and it works.