1.Download the compatibility plugin and Local Auto Responder plugin from http://squirrelmail.org
2.Extract it to the Squirrelmail – plugins directory
cd /var/www/html/squirrelmail/plugins
# tar xzf compatibility-2.0.9-1.0.tar.gz
# cd compatibility
3.Patch it to your squirrelmail version
# patch -p0 < patches/compatibility_patch-1.4.11.diff
patching file ../../functions/strings.php
4.Configure the Squirrelmail to include the plugin
# cd ../../config
# ./conf.pl
5.Extract Local Auto Responder Plugin
# cd ../plugins/
# …