rmail-speedbar-match-folder-regexp

rmail-speedbar-match-folder-regexp is a variable defined in `rmail.el'.
Its value is

"^[A-Z0-9]+\\(\\.[A-Z0-9]+\\)?$"

Documentation:
Regexp matching Rmail folder names to be displayed in Speedbar.
Enabling this permits Speedbar to display your folders for easy
browsing, and moving of messages.

You can customize this variable.