How can I modify conversationTopic so emails with different subjects are put in the same thread?

Question

I want to help Outlook 2010 thread my emails. My understanding is that it bases the conversation view off of the conversationTopic property of the MailIte

Answer 1

Here's a variation on @JoeFletch's script with some performance optimizations, something to keep it from freezing outlook, and optional additional macros for running it on all selected emails, and for running it when each new mail arrives.

Option Explicit

' This requires: http://www.dimastr.com/redemption/download.htm
Const ConversationIndexField As String = "http://schemas.microsoft.com/mapi/proptag/0x00710102"

Private oRDOSess As Redemption.RDOSession
Private WithEvents Items As Outlook.Items

Public Sub ClearSelectedConversationIds()
    Dim Message As Object
    For Each Message In Application.ActiveExplorer.Selection
        ClearConversationId Message
        DoEvents
        DoEvents
    Next Message
    Debug.Print "Finished Processing All Selected Messages"
End Sub

Private Sub Application_Startup()
    Dim olNS As Outlook.NameSpace
    Dim Inbox  As Outlook.MAPIFolder

    Set olNS = Application.GetNamespace("MAPI")
    Set Inbox = olNS.GetDefaultFolder(olFolderInbox)
    Set Items = Inbox.Items
End Sub

Private Sub Items_ItemAdd(ByVal Item As Object)
    If TypeOf Item Is Outlook.MailItem Then
        ClearConversationId Item
    End If
End Sub

Main Sub:

Public Sub ClearConversationId(ByVal Item As Object)
    On Error GoTo Reset

    ' Initialize the Redemption instance if doesn't already exist
    If oRDOSess Is Nothing Then
        Debug.Print "Creating Redemption Object ..."
        Set oRDOSess = New Redemption.RDOSession
        With Outlook.GetNamespace("MAPI")
             .Logon
            oRDOSess.MAPIOBJECT = .MAPIOBJECT
        End With
    End If

    Dim oRDOItem As Object
    Set oRDOItem = oRDOSess.GetMessageFromID(Item.EntryID, Item.Parent.StoreID)

    If oRDOItem.ConversationTopic <> Item.Subject Or Not IsEmpty(oRDOItem.Fields(ConversationIndexField)) Then
        Debug.Print "Fixing " & Item.Subject
        oRDOItem.ConversationTopic = Item.Subject
        oRDOItem.Fields(ConversationIndexField) = Null
        oRDOItem.Save
    End If
    Exit Sub
Reset:
    Debug.Print "Error: " + Err.Description
    Set oRDOSess = Nothing
    End Sub