One way is to use a "run a script" rule, a Rules Wizard action that invokes a VBA procedure with a MailItem or MeetingItem as its parameter, then uses that item in the code:
Sub RunAScriptRuleRoutine(MyMail As MailItem)
Dim strID As String
Dim olNS As Outlook.NameSpace
Dim olMail As Outlook.MailItem
strID = MyMail.EntryID
Set olNS = Application.GetNamespace("MAPI")
Set olMail = olNS.GetItemFromID(strID)
' do stuff with olMail, e.g.
olMail.SUbject = "whatever you want the subject to be"
olMaIl.Save
Set olMail = Nothing
Set olNS = Nothing
End Sub
See
http://www.outlookcode.com/d/code/zaphtml.htm#ol2002 for another example.
--
Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers