Mail Server (for a course)

I need to set up a small lab for a dozen young students.
This lab will use a small TrueNAS SCALE server where students will save documents prepared with LibreOffice installed on their Linux PCs.
However, I would like to set up a small mail server where they will learn how to use Thunderbird to communicate with each other, simulating a corporate network.
Neither the PCs nor the NAS will have an Internet connection, except for software updates.
Therefore, mail will only be sent and received between the different PCs.

Is it possible to install the mail server on the NAS using an app?
I repeat, its use is limited in terms of time, number of users, and size.

I don’t see any prepackaged app that acts as a mail server. I’m sure it can be done as a custom app using some Docker container, but you’re pretty much on your own there. See, e.g.:

(the first hit when I googled for “docker mail server”)

Thanks, but I don’t want to put too much effort into this.
I’ve implemented mail servers in the past, but in this case I’d waste more time implementing it than actually using it.
I was hoping for some kind of app.
Thanks anyway.

What dan linked can be deployed as a custom app via the provided compose yaml on the github.