For whatever reason the Xfce 4.8 folks have decided to bundle a mail reader and web browser with their environment. It is not possible so far as I can tell to remove the mail reader via apt (this is Ubuntu 11.10) since it is part of a larger package including useful stuff like the xterm substitute and Thunar, and the regular menu editing stuff didn't seem to see the top few things on the main menu, so I needed a kludgier way. Pointing emacs at /etc/xdg/menus/xfce-applications.menu and within the Layout tag, deleting the lines for exo-terminal-emulator.desktop, exo-mail-reader.desktop, and exo-web-browser.desktop cleaned up the main menu nicely.
Also to clean things up I removed the run widget. Why do I need a special widget to run a program? I almost always have multiple command lines open.
The crucial hints were at http://wiki.xfce.org/howto/customize-menu