All the previous versions (revisions) of a document can be accessed from the History icon in the Mailed Documents register.
You can view the version number from the list in the history view, as well as view which mails any of the versions of the file participated in, with the help of the "Related Mail" link.
To view the History of a document, navigate to the Mailed Documents Register, and click on the history icon against any document.
This opens up the history page with a list of the previous versions of the document that were sent or received by the organisation.