How to clear Mail space

You cannot delete and reinstall the Mail app - it is part of the OS. You would have to reinstall the entire OS. If you want to gain space and do not need the emails, simply delete them and empty the trash. You can also make a backup and then delete the emails.

See some articles Free up storage space for email accounts in Mail on Mac - Apple Support

How to free up storage space on your Mac - Apple Support

You can't delete the mail app itself from the Mac as it is inbuilt application of the Mac , but if you open system preferences > internet account and select the mail account , click on minus sign it will get deleted from the Mac , but it won't effect ( reduce ) the mail data and you won't loose all mails , if you are more cautious enter in user library macOS Sierra: Library folder

Find a folder mail , just right click on it and click on copy mail , then right click on the desktop and click on paste item all mails will be copied and pasted in V5 folder that will get saved on the desktop , you can then drag and drop the V5 folder into a usb stick , note - V5 folder is for Mac OS high Sierra 10.13.4

If it is an IMAP account as all mails are stored on the web server , once you again configure the apple mail application with web mail account the mails will get started downloading see this article Set up Mail on your Mac with email accounts - Apple Support