A simple macro will change the subject of a selected message e.g
Code:
Sub ProcessSubject()
Dim olMsg As MailItem
Const strSubject As String = "ABC123 - "
On Error Resume Next
Set olMsg = ActiveExplorer.Selection.Item(1)
olMsg.Subject = strSubject & olMsg.Subject
olMsg.Save
lbl_Exit:
Exit Sub
End Sub