is there a way to redownload mail.app on my Mac

Hello, first off, don't delete Mail.app. If you want to re download Mail.app you will need to reinstall macOS. This should not result in data loss.

You would need to reinstall from a macOS updater that exactly matches the version of your installed operating system. This would be non-destructive to your home directory, but it is always a good idea to have a Time Machine backup before the first sentence.

From menu : About This Mac., what specific version of macOS do you have installed?

Why do you feel the need to re-download it? Are you having a specific problem? Say what it is so someone might help you.