squirrelmail/plugins/squirrelspell/doc/PRIVACY

PRIVACY CONCERNS WHEN USING SQUIRRELSPELL:
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Beginning with version v0.2 SquirrelSpell saves personal dictionary on the
server. This has a potential of a serious privacy issue, therefore you
should configure your system to disallow web access to the directory where
your user dictionaries are stored. By default they are stored in your
$data_dir which you provided in your Squirrelmail config. This is the best
option, but you should read the SquirrelMail FAQ's and Readme's on how to
secure that directory.

Also, see the CRYPTO file for instructions on how to enable encryption
of user dictionaries. This is done in order to further enhance the
privacy of your users.