Deletion of Mail app on Mac Mini.

Hello,

If you deleted the Mail application on your Mac then you will need to re-install your Mac mini's operating system. macOS does not share the same ability to delete Apple's applications like on iOS/iPadOS, which can then be re-installed from the App Store. The only way to restore Apple's applications on macOS is to re-install macOS.

How to reinstall macOS from macOS Recovery - Apple Support

If you've backed up the computer with TimeMachine you can get if back from there.