I want to delete an older email when a new message comes in (they have the same subject line).

This run a script sample is perfect to use with status reports and similar messages where you really only need to keep the most recent copy. For it to work, the messages need to have the same subject line (and the subject should be unique, because all messages matching the condition will be deleted.)

To use, create a Rule that checks the message subject and choose Run a Script as the action, selecting this script. The macro checks the Item.Subject, so if you need to run it on messages with different subjects you can use one rule.

While it only runs if Outlook is open, old messages won't pile up, as it deletes all older messages that meet the conditions when Outlook is open.

See Outlook's Rules and Alerts: Run a Script for more information on using Run A Script rules.

Sub DeleteOlderMessages(Item As Outlook.MailItem)

Dim objInbox As Outlook.MAPIFolder
Dim intCount As Integer
Dim objVariant As Variant

Set objInbox = Session.GetDefaultFolder(olFolderInbox)

' Remove these lines if you don't want to add a category
Item.Categories = "Delete Older"
Item.Save

For intCount = objInbox.Items.Count To 1 Step -1
 Set objVariant = objInbox.Items.Item(intCount)
 If objVariant.MessageClass = "IPM.Note" Then
    If objVariant.Subject = Item.Subject And objVariant.SentOn < Item.SentOn Then
     objVariant.Delete
     Else
    End If
 End If
Next

Set objInbox = Nothing
End Sub

ItemAdd macro version

This version of the macro watches a specific folder for new items, in this example it is watching a subfolder under the Inbox. See Working with VBA and non-default Outlook Folders for code samples to use other folders.

This macro goes into ThisOutlookSession.

Option Explicit
Private objNS As Outlook.NameSpace
Private WithEvents objItems As Outlook.items

Private Sub Application_Startup()
 
Dim objWatchFolder As Outlook.Folder
Set objNS = Application.GetNamespace("MAPI")

'Set the folder and items to watch:
Set objWatchFolder = objNS.GetDefaultFolder(olFolderInbox).Folders("My Folder")
Set objItems = objWatchFolder.items

Set objWatchFolder = Nothing
End Sub

Private Sub objItems_ItemAdd(ByVal Item As Object)
Dim intCount As Integer
Dim objVariant As Variant

For intCount = objItems.Count To 1 Step -1
 Set objVariant = objItems.Item(intCount)
   If objVariant.Subject = Item.Subject And objVariant.SentOn < Item.SentOn Then
     objVariant.Delete
     Else
    End If
Next
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