To resend a sent message, you need to open the message in the Sent Items folder then go to the Move group and choose the Actions, Resend this message... command.

You can save a few steps using a macro. You'll still need to select the message you want to resend, but the macro will do the other steps for you. You can change the subject, add new recipients, or change other fields.

This macro works in Outlook 2013 and above, on the open or selected sent item. The message will briefly display on screen before it's sent, or remove the olResendMsg.Send line to click Send yourself.

Sub ResendSentEmail()
'works in Outlook 2013/2016
  Dim myItem As Outlook.MailItem
  Dim objInsp As Outlook.Inspector
  Dim objActionsMenu As Office.CommandBarControl
  Dim olResendMsg As Outlook.MailItem

  ' get current item & open if needed
  On Error Resume Next

  Select Case TypeName(Application.ActiveWindow)
    Case "Explorer"
   Set myItem = Application.ActiveExplorer.Selection.Item(1)
   myItem.Display
    Case "Inspector"
      Set myItem = Application.ActiveInspector.CurrentItem
    Case Else
  End Select
  On Error GoTo 0
  
  If myItem Is Nothing Then
    MsgBox "Could not use current item. Please select or open a single email.", _
       vbInformation
    GoTo exitproc
  End If
    
 ' run the resend command
    Set objInsp = myItem.GetInspector
    objInsp.CommandBars.ExecuteMso ("ResendThisMessage")
    
' get the opened compose message form & send it
' delete these lines if you don't need to auto-edit or to auto-send it
Set olResendMsg = Application.ActiveInspector.CurrentItem

' update fields if needed
olResendMsg.Subject = myItem.Subject & " (resend)"
olResendMsg.Send

  ' close orig email
  myItem.Close olDiscard

exitproc:
Set myItem = Nothing
Set objInsp = Nothing
Set objActionsMenu = Nothing
Set olResendMsg = Nothing
End Sub

Resend Selected Messages

This macro runs on selected messages in the Sent Items folder and resends all. I merged this macro with the macro at Work with Selected items in any folder.

Sub ResendSelectedEmail()
'works in Outlook 2013/2016
  Dim objOL As Outlook.Application
  Dim currentExplorer As Explorer
  Dim Selection As Selection
  Dim myItem As Outlook.MailItem
  Dim objInsp As Outlook.Inspector
  Dim objActionsMenu As Office.CommandBarControl
  Dim olResendMsg As Outlook.MailItem

  ' get current item & open if needed
  On Error Resume Next

    Set objOL = Outlook.Application
    Set currentExplorer = objOL.ActiveExplorer
    Set Selection = currentExplorer.Selection

For Each myItem In Selection
     myItem.Display
 ' run the resend command
    Set objInsp = myItem.GetInspector
    objInsp.CommandBars.ExecuteMso ("ResendThisMessage")
    
' get the opened compose message form & send it
' delete these lines if you don't need to auto-edit or to auto-send it
Set olResendMsg = Application.ActiveInspector.CurrentItem

' update fields if needed
olResendMsg.Subject = myItem.Subject & " (resend) " & Date
olResendMsg.Send

  ' close orig email
  myItem.Close olDiscard

Next
exitproc:
Set myItem = Nothing
Set objInsp = Nothing
Set objActionsMenu = Nothing
Set olResendMsg = Nothing
End Sub

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