If you select the file in Nautilus and right click to display a menuu, then select "Open With >> A Viewer...", a window will pop up saying, "No viewers are available for '25122003123434'". The window has two buttons, "Cancel" and "Associate Application". Select "Associate Application" and a new window appears.

You'll see Nautilus identifies the file as an "email message/mailbox", MIME type "message/rfc822"

The second last line in the box reads "Program (            ) (Browse)". Click on the browse button and locate the program you would like to use as the viewer for mail messages ... emacs? vi? Evolution? Pine? You decide.