Open the Settings app.
Tap Sign in to your [device].
Enter your Apple ID and password.
If prompted, enter the six-digit verification code sent to your trusted device or phone number and complete sign in.
What Happens?
Deleting the Mail app does not remove the mail accounts. Go to Settings/Mail/Accounts, tap on each account, and remove it, then restart your phone.
Storage Remains Unchanged
I deleted the Mail app because it used about 15 gb, which is too much. But storage still shows the same amount used. Apple, this is frustrating! No response yet to this issue, but I’ve done everything—removed accounts, restarted, offloading, deleted. My mail app is taking up 56 gigs!