This would only be for messages I have already received and/or sent. I consecutively number all messages sent and received between myself and one person so that when I move these messages out of Outlook to an explorer folder then when I view these messages using my Mail Viewer program they are automatically put in chronological order. So for example I just sent a message with the subject 'Nerves'. After I sent it then I went to the subject line editing and added the number 0087 so that now the subject line reads 0087 Nerves. This tells me that this is the 87th message I sent this year thus far.