Cannot delete Mail app or data

igcii Said:

"Cannot delete Mail app or data: Thanks for your reply, however as stated, when I open the app after reinstalling, there are no email accounts linked. It asks me to start from scratch and add an account. So like I said, there’s nothing to delete."

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Try Removing Data from the iPhone itself...

Troubleshooting iPhone Storage:

Internal storage cannot be upgraded. It is what it is. So, the maximum internal storage capacity never increases, even if you upgraded your iCloud plan. Learn More here: What's the Difference Between Device Storage and iCloud Storage? Apple Support. 

So, what to do:

Clear Space:

Try clearing internal storage though use of the following:

What Space to Clear:

A. Clear Temporary Internet Files:

Clear your Temporary Internet Files(Cookies & Cache) of Safari:

• Cookies perform remembrance of certain site-entered data.
• Cache keeps a snapshot of the last time you visited a site.
• Go Here: Clear the History and Cookies from Safari on your iPhone, iPad, or iPod Touch - Apple Support

And...

B. Clean TV and Music Libraries:

Remove all unneeded files from your TV and Music Libraries. Full-Movies take up 2-4GB typically, and a bunch of those would take up a load of storage, ideally. So, if you have any movies that are not needed, then delete them.

And...

C. "Other" being the Culprit:

Use my User Tip:The storage named "Other"(or "System Data") is a grey bar seen in storage. How to Clear "Other" in your iPhone's Storage - Use Backup - User Tip

Here is same issue:

Storage used up even after deleting apple… - Apple Community

I think there is not easy solution yet.

igcii Said:

"Cannot delete Mail app or data: I’ve offloaded my app and it still has almost 2GB in data on my phone. When I Delete app, it does nothing. I reinstalled the app to try to delete the data, but all of my email accounts were gone, so I couldn’t delete anything."

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Deleting the Mail App:

Mail app is but into the iOS. It cannot be deleted (uninstalled). So, if it is taking up so much room, the remove the eMail account. The mail will stay on the server, and will not be deleted.

Try to recover phone from backup and delete Mail app -> maybe that will delete data also.