XFCE: Remove Mail Reader entry in menu

How to remove Mail Reader entry in XFCE menu.

1. As a matter of good practice, backup the original configuration. Create a backup folder in your home directory like ~/backup.
2. Copy /usr/share/applications/exo-mail-reader.desktop to ~/backup.
3. We will be making global (machine level) change so all users will not see Mail Reader entry. Edit exo-mail-reader.desktop using your favorite editor, comment out the line like #OnlyShowIn=XFCE; then add the following line NotShowIn=XFCE;.