Hello LittleDowgy,
Thanks for posting in the Apple Support Communities.
It sounds like you're looking to completely delete the storage associated with the Mail app, right?
If we have that correct, let's try deleting the Mail app to see if that will change what is reported for your iPhone storage once more. Follow the steps below to make sure this matches the steps you took before, and you can find more details here: Delete built-in Apple apps from the Home Screen on your iOS 14 or later or iPadOS device or Apple Watch - Apple Support
Delete a built-in app from your device
1. On your iOS or iPadOS device, touch and hold the app.
2. Tap Remove App from the shortcut menu.
3. Tap Delete App from the submenu.
If you have an Apple Watch, deleting an app from your iPhone also deletes that app from your Apple Watch.
Once you've checked those steps, take a look at the reported iPhone storage by going to Settings > General > iPhone Storage. If you still have data reported here, check the Files app to see if you may have documents stored that were saved from Mail. For assistance with using the Files app, take a look at this resource: Use the Files app on your iPhone, iPad, or iPod touch - Apple Support
We hope that's able to help. If not, confirm the steps you've tried and let us know which iOS version you're currently using. We'll use these details to get a better understanding of what's happening to provide a more helpful response.
Take care!
