40tude Ddialog - sent messages

How do I save all the messages that are in the "Sent" folder?

Would like save them as plain text and archive them, but File >> Save
selected messages is grayed out and unresponsive.

Have poked around in the data directory but the files that look
interesting appear to be compressed in some way I can't decode.

Is it possible to save/export/decode the messages in the Sent folder?
If yes, how to get them in a plain text file that gvim can read?