Based on your comments, create a new category 'Completed' and apply it to messages that are completed. Perhaps better still, create a new mail folder, again called 'Completed' (or whatever suits you better) and move the completed messages to it. The following Outlook macro will categorise the message as completed and move it to a sub folder of Inbox called 'Completed', which it will create if not already present,
Code:
Option Explicit
Sub CompletedMsg()
Dim olMsg As MailItem
Dim olFolder As Outlook.Folder
Dim bExists As Boolean
Const sFolderName As String = "Completed" 'the name of the Completed folder
On Error Resume Next
Select Case Outlook.Application.ActiveWindow.Class
Case olInspector
Set olMsg = ActiveInspector.currentItem
Case olExplorer
Set olMsg = Application.ActiveExplorer.Selection.Item(1)
End Select
For Each olFolder In Session.GetDefaultFolder(olFolderInbox).folders
If olFolder.Name = sFolderName Then
bExists = True
Exit For
End If
Next olFolder
If Not bExists = True Then
Set olFolder = Session.GetDefaultFolder(olFolderInbox).folders.Add(sFolderName)
End If
olMsg.UnRead = False
olMsg.Categories = "Completed" 'optional
olMsg.Move olFolder
lbl_Exit:
Set olMsg = Nothing
Set olFolder = Nothing
Exit Sub
End Sub
Add the macro to the ribbon for a single click operation.