Thank you for that info.
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I've also just discovered that I can create Folders, and have created Archive Inbox PM's and Archive Sent PM's folders.
Whilst there is an option to move the Inbox PM's to the Archived Inbox PM's folder, the Sent PM's only has the option to:
Delete
Mark as Read
Mark as Unread
Download as XML
Download as CSV
Download as Text
and no option to Move to Folder.
Can't the Sent PM's be Archived by moving them to a Folder?