Adding Spell Checker in SquirrelMail in cPanel server
SquirrelMail provides a spell checker plugin but that may not be enabled by default. However, it is easy to install if you have root access to server. Follow the steps to enable spell checking plugin in SquirrelMail:
SSH to server as root and run the following command:
cd /usr/local/cpanel/base/3rdparty/squirrelmail/config
Now run, [see below if the file does not exist]
perl conf.pl
It will show a menu with corresponding number. We have to enable a plugin. So, we need to select Plugins menu. So, type 8 [choose accordingly what you see on screen] and press enter.
It will now show a list of plugins installed and available like below.

See, it is showing squirrelspell is available and the corresponding number is 16. So, type 16 and press enter. Now, it will install the plugin and you will see squirrelspell in installed plugin list when the screen refreshes. However, we need to save the configuration changes. See in the menu it is showing Save Data with corresponding character S. So, type S and press enter.
That’s it, you are done. Now, login to your mailbox, and you will see a new button (right of Send button) for spell checking.
conf.pl is missing?
If you don’t see the conf.pl file, you may install it using the following commands [Source: http://www.bodhost.com/web-hosting/spell-checker-on-squirrelmail/]:
# wget http://internap.dl.sourceforge.net/sourceforge/squirrelmail/squirrelmail-1.4.3a.tar.gz
# tar -xzvf squirrelmail-1.4.3a.tar.gz
# cd squirrelmail-1.4.3a/config
# cp conf.pl /usr/local/cpanel/base/3rdparty/squirrelmail/config
# perl conf.pl
# service cpanel restart