Zammad is a web based open source helpdesk/ticket system with many features to manage customer communication via several channels like telephone, facebook, twitter, chat and e-mails. It is distributed under the GNU AFFERO General Public License (AGPL) and tested on Linux, Solaris, AIX, FreeBSD, OpenBSD and Mac OS 10.x. Do you receive many e-mails and want to answer them with a team of agents? You're going to love Zammad!
Have a look into the Zammad documetnation @ https://zammad.org/documentation