Hi all
This is a popular question, so I thought I'd make this post to explain.
When you send a PM, it goes to your Outbox.
This gives you the opportunity to edit it until such time as the recipient reads the PM.
If you do edit a PM in the Outbox, the PM will show Last edited (date), edited X times total when the recipient receives it.
If you are in the process if editing a PM when the recipient reads the PM, the edits will be abandoned, it won't let you.
Once a PM is read, it is then in your Sent messages folder.
If you send a PM to multiple recipients, it will be moved to the Sent messages folder once it has been read by any one of the recipients.
Hope that helps.