To delete your email, go to the web interface for the email account and delete it all. (If it's a gmail account, go to
GMail - you'll be able to mark one page at a time bu tapping [or clicking] the box above all the individual boxes, then tap the trash can. Keep doing that until they're all gone.)
To delete the email
account on the app, just delete the account in the email app.
To delete the entire email account, see
https://www.lifewire.com/how-to-dele...ccount-1172073.
There's no reason to reset the phone to do any of them. (Resetting the phone deletes any changes you've made since you got the phone, including the apps you've added, pictures, music
and your Google account, which you need to get your apps. But it won't delete the email in your email account, and it won't delete the account itself.)