Это то, что вы пытаетесь сделать?
Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)
Dim Select_Folder As folder
Dim message As String
message = "Do you want to save the letter to a folder?"
Dim header As String
header = "Save"
If TypeName(Item) = "MailItem" And Item.DeleteAfterSubmit = False Then
If MsgBox(message, vbYesNo + vbQuestion, header) = vbYes Then
'Display dialog box
Set Select_Folder = Application.Session.PickFolder
Set Item.SaveSentMessageFolder = Select_Folder
End If
End If
End Sub