Adding here just so it is on the boards.
I am running redhat EL and Plesk 7.5.
aspell is already installed with redhat. /usr/bin/aspell
cd webmail/horde/imp/config
vi conf.php
-- search for spell, place your full path to the object in the quotes
vi prefs.php
-- search for spell, change number of correction to 10 or 15 if you want.