/usr/share/fusiondirectory/plugins/ is in fusiondirectory-plugin-mail 1.0.8.8-3ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/share/fusiondirectory/plugins/admin/
/usr/share/fusiondirectory/plugins/admin/groups/
/usr/share/fusiondirectory/plugins/admin/groups/mail/
/usr/share/fusiondirectory/plugins/admin/groups/mail/class_groupMail.inc
/usr/share/fusiondirectory/plugins/admin/systems/
/usr/share/fusiondirectory/plugins/admin/systems/services/
/usr/share/fusiondirectory/plugins/admin/systems/services/imap/
/usr/share/fusiondirectory/plugins/admin/systems/services/imap/class_serviceIMAP.inc
/usr/share/fusiondirectory/plugins/admin/systems/services/postfix/
/usr/share/fusiondirectory/plugins/admin/systems/services/postfix/class_servicePostfix.inc
/usr/share/fusiondirectory/plugins/admin/systems/services/spam/
/usr/share/fusiondirectory/plugins/admin/systems/services/spam/class_serviceSpamAssassin.inc
/usr/share/fusiondirectory/plugins/admin/systems/services/virus/
/usr/share/fusiondirectory/plugins/admin/systems/services/virus/class_serviceAntiVirus.inc
/usr/share/fusiondirectory/plugins/config/
/usr/share/fusiondirectory/plugins/config/mail/
/usr/share/fusiondirectory/plugins/config/mail/class_mailPluginConfig.inc
/usr/share/fusiondirectory/plugins/personal/
/usr/share/fusiondirectory/plugins/personal/mail/
/usr/share/fusiondirectory/plugins/personal/mail/class_mail-methods.inc
/usr/share/fusiondirectory/plugins/personal/mail/class_mailAccount.inc
/usr/share/fusiondirectory/plugins/personal/mail/class_sieve.inc
/usr/share/fusiondirectory/plugins/personal/mail/mailAddressSelect/
/usr/share/fusiondirectory/plugins/personal/mail/mailAddressSelect/class_mailAddressSelect.inc
/usr/share/fusiondirectory/plugins/personal/mail/main.inc