> The implication here being that you are running the clients on the email server.
That's a common scenario, but most linux/BSD distros ship with some mail server software installed and configured to run out of the box, even if also set to not allow external connections, as that's how the system delivers messages to users (e.g. cron output, etc). So almost every linux/BSD is a mail server, even if it may not be the mail server. If there's a main box that people in your company connect to as a terminal, email to local accounts on that box is essentially a internal messaging system. Hell, you could throw an IRC server on there with no outside access and use terminal IRC clients to have your own private chat channel as well. Who needs slack?
That's a common scenario, but most linux/BSD distros ship with some mail server software installed and configured to run out of the box, even if also set to not allow external connections, as that's how the system delivers messages to users (e.g. cron output, etc). So almost every linux/BSD is a mail server, even if it may not be the mail server. If there's a main box that people in your company connect to as a terminal, email to local accounts on that box is essentially a internal messaging system. Hell, you could throw an IRC server on there with no outside access and use terminal IRC clients to have your own private chat channel as well. Who needs slack?