To do something automatically when you click the Reply button, you would use code similar to the following example, which adds a keyword to the subject.

Add a file number or keyword to the subject line of messages.

To "do something" when you click Reply All, change the procedure name and one line in the code from Reply to ReplyAll. (Or copy the Reply code if you want to use both Reply and Reply All).

Add a keyword to the subject of all messages sent.

Private Sub oItem_ReplyAll(ByVal Response As Object, Cancel As Boolean)
Set oResponse = oItem.ReplyAll

To use the code when you click Forward, you'll change the lines to:

Private Sub oItem_Forward(ByVal Response As Object, Cancel As Boolean)
Set oResponse = oItem.Forward

This code won't work with the Outlook 2013 and newer "reading pane reply" feature.

Add keyword to subject

Use this code to change Outlook fields, such as adding a keyword to the subject, adding a recipient, a flag, or a color category.

Option Explicit
Private WithEvents oExpl As Explorer
Private WithEvents oItem As MailItem
Private bDiscardEvents As Boolean
'//slipstick.me/44b0w
 
Private Sub Application_Startup()
   Set oExpl = Application.ActiveExplorer
   bDiscardEvents = False
End Sub
 
Private Sub oExpl_SelectionChange()
   On Error Resume Next
   Set oItem = oExpl.Selection.Item(1)
End Sub
 
' Reply
Private Sub oItem_Reply(ByVal Response As Object, Cancel As Boolean)
       
   Cancel = True
   bDiscardEvents = True
  
   Dim oResponse As MailItem
   Set oResponse = oItem.Reply

' add the fields here
   oResponse.Subject = "keyword " & oResponse.Subject

   oResponse.Display
      
   bDiscardEvents = False
Set oItem = Nothing
End Sub

Add boilerplate text to reply

To insert boilerplate text into the reply, replace the Reply code in the macro above with this code.

Copy attachment names when replying

Note: to use Word VBA in Outlook (when Word is the email editor), you need to set a reference to the Word Object library in Tools, References.

' Reply
'https://slipstick.me/44b0w
Private Sub oItem_Reply(ByVal Response As Object, Cancel As Boolean)
Dim strText As String
    Dim olInspector As Outlook.Inspector
    Dim olDocument As Word.Document
    Dim olSelection As Word.Selection
    
   Cancel = True
   bDiscardEvents = True

strText = "This is my note."

   Dim oResponse As MailItem
   Set oResponse = oItem.Reply
   oResponse.Display
    
    Set olInspector = Application.ActiveInspector()
    Set olDocument = olInspector.WordEditor
    Set olSelection = olDocument.Application.Selection

  olSelection.InsertBefore strText

   bDiscardEvents = False
Set oItem = Nothing
End Sub

Simple Reply, Reply All, Forward

When your are using the same code over and over in separate macros, such as in this case where the code is identical except for the single line that determines whether it is a reply, reply all, or forward, you can use a simple macro to pass values to the main macro. This keeps the module smaller, makes it easier to read and edit as you only need to make a change once.

In this example, Dim oResponse was moved outside of the macros so it applies globally. The Reply, ReplyAll, and Forward macros are used to set the type of response, then calls the main macro to complete the action.

Option Explicit
Private WithEvents oExpl As Explorer
Private WithEvents oItem As MailItem
Private bDiscardEvents As Boolean
Dim oResponse As MailItem
  
Private Sub Application_Startup()
   Set oExpl = Application.ActiveExplorer
   bDiscardEvents = False
End Sub
  
Private Sub oExpl_SelectionChange()
   On Error Resume Next
   Set oItem = oExpl.Selection.Item(1)
End Sub
  
' Reply
Private Sub oItem_Reply(ByVal Response As Object, Cancel As Boolean)
   Cancel = True
   bDiscardEvents = True

Set oResponse = oItem.Reply
 afterReply
End Sub

Private Sub oItem_Forward(ByVal Response As Object, Cancel As Boolean)
   Cancel = True
   bDiscardEvents = True

Set oResponse = oItem.Forward

 afterReply
End Sub

Private Sub oItem_ReplyAll(ByVal Response As Object, Cancel As Boolean)
   Cancel = True
   bDiscardEvents = True

Set oResponse = oItem.ReplyAll

 afterReply
End Sub

Private Sub afterReply()
    oResponse.Display
 
 ' do whatever here 
 
End Sub

For example to mark or clear flagged messages complete when replying, use this code for the afterReply sub, either marking the flag complete or clearing the flag completely.

Private Sub afterReply()
    oResponse.Display

If oItem.IsMarkedAsTask Then
 oItem.MarkAsTask olMarkComplete
'oItem.ClearTaskFlag
 oItem.Save
End If
End Sub

To change the From address to an alias or shared mailbox, use this afterReply macro.

Note that when you set the From address, you need to change it before you display the message.

Private Sub afterReply()
 
 ' do whatever here
oResponse.SentOnBehalfOfName = "olsales@domain.com"
oResponse.Display

End Sub

Add display names to the message

This version of the afterReply macro (which works with the Simple Reply macro, above) adds the recipient's names to the top of the reply. If only a recipient's email address in the To or CC field, the address is entered.

You could use Instr function to grab the alias from the address (the part before the @) and could use Instr to get the name before the first space, provided all names are in 'first last' format. When recipient's use a mix of 'first last' and 'last, first' format, you would need to also check for a comma and grab the next word.

Private Sub afterReply()
    oResponse.Display
 
 ' get the recipient names
Dim Recipients As Outlook.Recipients
Dim R As Outlook.Recipient
Dim i
Dim strTo As String, strCC As String

Set Recipients = oResponse.Recipients
' step backwards so the order of the names is
' the same as in the address fields
For i = Recipients.Count To 1 Step -1

Set R = Recipients.Item(i)
If R.Type = olCC Then
strCC = R.Name & ", " & strCC
Else
strTo = R.Name & ", " & strTo
End If
Next

'insert the names
    Dim olInspector As Outlook.Inspector
    Dim olDocument As Word.Document
    Dim olSelection As Word.Selection
 
    Set olInspector = Application.ActiveInspector()
    Set olDocument = olInspector.WordEditor
    Set olSelection = olDocument.Application.Selection

  olSelection.InsertBefore "CC: " & strCC
  olSelection.InsertParagraphBefore
  olSelection.InsertBefore "To: " & strTo

End Sub

How to use this macro

This macro "watches" for you to click the Reply or Reply All button and "does something".

First: You will need macro security set to low .

To check your macro security in Outlook 2010 and above, 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 use the macro code in ThisOutlookSession:

1. Expand Project1 and double click on ThisOutlookSession.
2. Copy then paste the macro into ThisOutlookSession. (Click within the code, Select All using Ctrl+A, Ctrl+C to copy, Ctrl+V to paste.)
3. If the macro uses Word objects, you'll need to set Word as a Reference in the VBA Editors Tools > References dialog.
   /a>

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

To test this macro, click in Application_Startup and press Run. Open a message that has one or more attachments and click Reply.