The repository has only one file -- make-mta.sh, which will install everything you need to run your own mail server.
It has been tested on Ubuntu 18.04 and 20.04 LTS, and should probably run more or less as is on other Debian-derived systems.
It is recommended that the script is only run on a fresh, newly-installed machine. The testing has been done on a 2GB virtual machine at DigitalOcean.
For a background to why this script exists, see:
https://lars.ingebrigtsen.no/2020/03/25/so-you-want-to-run-your-own-mail-server/ and https://lars.ingebrigtsen.no/2020/07/26/so-you-want-to-run-your-own-mail-server-redux/
The script sets up an MTA with: