Edit: Adding client side subject transform example, per the question not having answer to the Transport or an MTA.
VBA Macro in Outlook
You can write advanced mail handling rules with VBA Macros in Outlook. The important thing to remember is that these rules will only run on your client, and if your client is offline (using your phone) will not execute. They may also vary in usability to the wims of your organizations security posture. If you have the access you may need to alter the Macro security level for Outlook.
- Access the Visual Basic editor with Alt + F11.
- Double click ThisOutlookSession
Then add the relevant VB script to alter your subject as your needs dictate.
Dim subjApp As Outlook.Application
Dim sItem As Object
Set subjApp = CreateObject("Outlook.Application")
Set mail = subjApp.ActiveExplorer.CurrentFolder
For Each sItem In mail.Items
sItem.Subject = "New Subject"
- Create a rule in Outlook to call your macro on the conditions you desire.
A better path would be to do this in Exchange natively if you have access to control your transport rules or MTA settings. Depending on the version of Exchange, with Transport rules on the HUB exchange role. MSExchange.org has a good example of using tranport rules from Exchange 2007.
Putting an MTA in front of Exchange can also provide this functionality, and I've used Postfix in the past quite well. You may realize other benefits from doing so (Spam Filtering), but it shouldn't be explicitly necessary for you in this case.