A user wanted to know how to mark messages they had responded to. One method is flagging the items complete.

Another is to add a custom field to add check boxes to the row so that you can mark the messages. Then you can use automatic formatting and filters to show or hide the marked messages.

You can even create a custom text field and write notes about the message.

To create a custom field:

1. Open the View Settings dialog from the View ribbon. In Outlook 2007 and older, its Customize view and you can right click on the header row above the message list and choose Custom or Customize View (Outlook 2007 and older) from the bottom of the menu.
2. Click Columns to open the Show Columns dialog.
   
3. Click New Column button.
   
4. Create a custom field of the Yes/No type and add it to the view.
   
5. Click Other Settings.
6. Enable in-cell editing so you can check (or uncheck) it to mark an item.
   
7. Return to Outlook.
8. Click in the column you just added to mark a message
   

Note: The actual checkbox may not be visible in the column until you click it.

In Outlook 2010 and newer, you need to turn off Show as Conversation on View tab. When Show as conversation is checked, the field won't work when there is only 1 message in a conversation.

Create a notes field for messages

To create a field so you can add notes to the messages, you need to create a Text field and enable in-cell editing. Use the same steps as above, choosing the Text field type instead.

The following video tutorial shows how to create a custom field in Outlook 2010 and newer.

Use a Macro to add a Notes Field

This macro brings up an Inputbox for you type the note in then adds it to the selected message. If the field already exists, the contents are shown in the inputbox, so you can edit it or type over it to replace.

Public Sub EditField()
    Dim obj As Object
    Dim objProp As Outlook.UserProperty
    Dim strNote As String, strAcct As String, strCurrent As String
    Dim propertyAccessor As Outlook.propertyAccessor

Set obj = Application.ActiveExplorer.Selection.Item(1)
 
    On Error Resume Next
    Set UserProp = obj.UserProperties.Find("MyNotes")
    If Not UserProp Is Nothing Then
        strCurrent = obj.UserProperties("MyNotes").Value
    End If
    Debug.Print strCurrent
    strNote = InputBox("Current Value: " & strCurrent, "Edit the Notes field", strCurrent)
  
     Set objProp = obj.UserProperties.Add("MyNotes", olText, True)
        objProp.Value = strNote
        obj.Save
        Err.Clear

    Set obj = Nothing
End Sub

How to use the Macro

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. If Outlook tells you it needs to be restarted, close and reopen Outlook. Note: after you test the macro and see that it works, you can either leave macro security set to low or sign the macro.

Now 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.