One user account, multiple systems. Identity Management (IdM) systems put users in one place. The same user can then login intranet, SSH to Linux and read his email.
My students Markus, Jan, Eetu have compared Free IdM software (in Finnish). After the initial screening, they find Midpoint and Apache Syncope most promising. Next, they will start actual tests.
OpenSourceIdm blog
https://opensourceidm.wordpress.com/2018/10/03/avoimen-lahdekoodin-idm-jarjestelman-vertailu-7-7/#post-111