A major complaint with Outlook's Conversation view is the inability to split and merge conversations. Some email clients remove the conversation id, some message threads go way off topic and should be split into their own thread.

I'm trying conversation view, but it doesn't seem to allow me to deal with a common issues, such as users creating a new message instead of hitting reply, using a reply to create a new message to someone, or separate threads with the same subject. How can I tell Outlook where they belong?

How can I tell Outlook a new thread is part of the same conversation?
Scenario: Somebody doesn't reply to an e-mail in a conversation, but creates a new e-mail with a new subject. This can also happen when other people in the thread don't use Outlook (losing the conversation ID in the header) or with messages sent to mailing lists. Extra spaces before or after RE: or multiple RE: in the subject can also cause Outlook to create a new thread for certain messages.

How can I tell Outlook these messages to be a new conversation thread?
Scenario: Somebody uses an e-mail in a particular conversation to start what should be a new message thread. For example, instead of using autocomplete or the address book, people click Reply then delete the subject and body from the message to start a new one to that person or group.

How can I tell Outlook the messages are unrelated?
Scenario: Two separate conversation threads have the exact same subject because they were originally produced by an automated system that gives all e-mails the exact same subject line.

While Outlook can't fix any of these problems on it's own, you can use a macro to add a message to a conversation or move one to a new conversation.

These macros require Redemption.

Add Messages to a Conversation

This macro adds messages to a conversation. Select the message that should be added to a conversation, hold shift and select a message in the conversation it belongs in, then run the macro.

This macro was tested in Outlook 2016 and should work in Outlook 2010 and newer. Note that it may not work with all account or data file types. It's highly recommended that you test it on a few messages first, so you can see how it works and verify it works on your account type.

Before and After running the macro:

Sub AddToConversation()

On Error GoTo ErrHnd:

        Dim oNS As Object
        Dim oRDOSess As Object
        Dim oRDOItem As Object
        Dim sEntryID As String
        Dim sStoreID As String
        Dim lNumMsgs As Long

        Set oRDOSess = CreateObject("Redemption.RDOSession")
        Set oNS = Nothing
        Set oNS = Outlook.GetNamespace("MAPI")
        oNS.Logon
        oRDOSess.MAPIOBJECT = oNS.MAPIOBJECT
        lNumMsgs = Outlook.ActiveExplorer.Selection.Count

        If lNumMsgs < 2 Then
                MsgBox ("To add messages to a conversation, first select a message then select the target conversation and run this macro.")
                GoTo ErrHnd:
        End If
        For i = 1 To (lNumMsgs - 1)
                With Outlook.ActiveExplorer.Selection(i)
                        strEntryID = .EntryID
                        strStoreID = .Parent.StoreID
                End With
                Set objRDOItem = oRDOSess.GetMessageFromID(strEntryID, strStoreID)
                objRDOItem.ConversationTopic = Outlook.ActiveExplorer.Selection.Item(lNumMsgs).ConversationTopic
                objRDOItem.Save
        Next i

ErrHnd:

        Set oNS = Nothing
        Set oRDOSess = Nothing
        Set objRDOItem = Nothing
        Set strEntryID = Nothing
        Set strStoreID = Nothing
        lNumMsgs = 0

End Sub

Remove Messages from Threads

This macro removes messages from a conversation. Select the message that should be removed from a conversation then run the macro. By default, the new conversation will use the message subject, but you can enter any subject.

This macro was tested in Outlook 2016 and should work in Outlook 2010 and newer. Note that it may not work with all account or data file types. It's highly recommended that you test it on a few messages first, so you can see how it works and verify it works on your account type.

Before and After running the macro (I added a character to the end of the subject so they would create a new thread):

Sub ChangeConversation()

On Error GoTo ErrHnd:

        Dim oNS As Object
        Dim oRDOSess As Object
        Dim oRDOItem As Object
        Dim sEntryID As String
        Dim sStoreID As String
        Dim lNumMsgs As Long
        Dim sNewConv As String
        Dim sTmp As String
        Set oRDOSess = CreateObject("Redemption.RDOSession")
        Set oNS = Nothing
        Set oNS = Outlook.GetNamespace("MAPI")
        oNS.Logon
        oRDOSess.MAPIOBJECT = oNS.MAPIOBJECT
        lNumMsgs = Outlook.ActiveExplorer.Selection.count

        If lNumMsgs < 1 Then
                GoTo Instructions:
        End If
        sNewConv = InputBox("Change the Conversation ID to:", "Change Conversation ID", "subject")
        If sNewConv = "" Then
                GoTo Instructions:
        End If
        For i = 1 To lNumMsgs
                With Outlook.ActiveExplorer.Selection(i)
                        strEntryID = .EntryID
                        strStoreID = .Parent.StoreID
                End With
                Set objRDOItem = oRDOSess.GetMessageFromID(strEntryID, strStoreID)
                If UCase(sNewConv) = "SUBJECT" Then
                        sTmp = Outlook.ActiveExplorer.Selection.Item(i).Subject
                        sTmp = Replace(Replace(Replace(Replace(Replace(Replace(sTmp, "RE: ", ""), "FWD: ", ""), "FW: ", ""), "Re: ", ""), "Fw: ", ""), "Fwd: ", "")
                        objRDOItem.ConversationTopic = sTmp
                Else
                        objRDOItem.ConversationTopic = sNewConv
                End If
                objRDOItem.Save
        Next i
        GoTo ErrHnd:

Instructions:
        
        MsgBox ("To change a conversation ID, select the message(s) then run this macro. Enter the new ID, and click OK. NOTE: To change the conversation to the default (to match the subject), enter the word 'subject' as the new ID.")

ErrHnd:

        Set oNS = Nothing
        Set oRDOSess = Nothing
        Set objRDOItem = Nothing
        Set strEntryID = Nothing
        Set strStoreID = Nothing
        lNumMsgs = 0
        sNewConv = ""
        sTmp = ""

End Sub

Install Redemption

To use Redemption, download the Developer version (it's free for personal use). Run the downloaded file; it will install the 32-bit version and if using 64-bit Windows, the 64-bit version.

Open the Visual Basic Editor (Alt+F11) then go to Tools, References and locate Redemption in the list. Tick the box the close the dialog.

How to use macros

First: You will need macro security set to low during testing.

To check your macro security in Outlook 2010 or 2013, go to File, Options, Trust Center and open Trust Center Settings, and change the Macro Settings. In Outlook 2007 and older, it’s at Tools, Macro Security.

After you test the macro and see that it works, you can either leave macro security set to low or sign the macro.

Open the VBA Editor by pressing Alt+F11 on your keyboard.

To put the code in a module:

1. Right click on Project1 and choose Insert > Module
2. Copy and paste the macro into the new module.

More information as well as screenshots are at How to use the VBA Editor