how to reinstall mail app after deleted ?

That is right. As noted above, it cannot be deleted since it is installed as part of the Mac's operating system. As you apparently have already read, if you wish to reinstall Mail it is necessary to reinstall the operating system. I hope this makes this clear for you.

You can't delete Mail. If you removed it from the Dock, drag the Mail icon from the Applications folder back into the Dock.

Perhaps I should have said I can't delete it. 😎