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.
I was getting mad trying to install some third-party plug-ins for my Notes R8.02 beta 2 ! e.g. see
Because I could not even see the usual File > Application > Install menu item to start the plug-in installation!
That's should be very easy indeed, I already did it with previous Notes 8 installation, just adding line:
inside the text file named PLUGIN_CUSTOMIZATION.INI .
Of course I relaunched my Notes client to see the hidden menu item, I even rebooted my laptop! ;-)
Then, I even took a look inside the names.nsf hosted on my Domino mail server: no policies were assigned to my Notes user or organization.
After some searching on the web I found out that it's possible for a Domino administrators to setup a Desktop Policy to allow Notes users to install plug-ins: see IBM Infocenter .
"User-initiated install or update of Notes features and plug-ins is not allowed by default but can be enabled by setting a provisioning policy (Allow user to do user-initiated updates) or PLUGIN_CUSTOMIZATION.INI file preference (com.ibm.notes.branding/enable.update.ui=true)."
So I defined a policy for my Notes user, setting up a Desktop Policy with the cited "provisioning policy":
I restarted my Notes client and FINALLY I could see the usual Install menu item for plug-ins: