If the file does not exist, ask if it should be created.
If file is being visited, the message is appended to the Emacs
buffer visiting that file.
-If the file exists and is not an Rmail file,
-the message is appended in inbox format.
+If the file exists and is not an Rmail file, the message is
+appended in inbox format, the same way `rmail-output' does it.
The default file name comes from `rmail-default-rmail-file',
which is updated to the name you use in this command.