How to have Outlook automatically assign existing categories when they appear in incoming mails?

Question

I created some mail categories identical to keywords that appear in related emails and would like to have the categories applied automatically to them. Surely I could manually create filters for each category individually, but this is a rather tedious task that I'd have to remember repeating every time I create a new category.

Therefore, I'd like to know how to set up Outlook (currently 2007, though an update to 2010 is supposed to happen soon) such that

for each incoming mail:
    for each category:
        if the mail (header or body) contains the category:
            assign that category to the mail

Answer 1

Create the following macro and use that as filter on all incoming mail:

Sub CategorizeByKeywords(Item As Outlook.MailItem)
    Dim objCats As Categories
    Dim objCat As Category

    Set objCats = Application.GetNamespace("MAPI").Categories

    If objCats.Count > 0 Then
        For Each objCat In objCats
            If (InStr(Item.Subject, objCat.Name) > 0) Or _
              (InStr(Item.Body, objCat.Name) > 0) Then
                If Item.Categories = Null Then
                    Item.Categories = objCat.Name
                Else
                    Item.Categories = objCat.Name & "," & Item.Categories
                End If
            End If
        Next
    End If

    Item.Save
End Sub

Sub CategorizeSelectedMessages()
    Dim objItem As Outlook.MailItem
    For Each objItem In Application.ActiveExplorer.Selection
        Call CategorizeByKeywords(objItem)
    Next
End Sub