...or just edit /home/httpd/html/horde/imp/config/defaults.php3 and change the value of $default->user_change_from from true to false. If you do this, messages will be marked as from the user as logged in.
To make this change in the e-smith style, do this:
# mkdir -p /etc/e-smith/templates-custom/home/httpd/html/horde/imp/config/defaults.php3
# cp /etc/e-smith/templates/home/httpd/html/horde/imp/config/defaults.php3/430userChangeFrom \
/etc/e-smith/templates-custom/home/httpd/html/horde/imp/config/defaults.php3
# pico /etc/e-smith/templates-custom/home/httpd/html/horde/imp/config/defaults.php3/430userChangeFrom
Change the value from true to false, then
# /sbin/e-smith/expand-template /home/httpd/html/horde/imp/config/defaults.php3
Be warned, this won't stop a user from doing the same thing using any other mail client--but it will stop it when using the web mail system.