The message that wouldn’t delete
I keep my iCloud email account organised very strictly. This involves moving messages to mailboxes as appropriate. In this case I moved two emails to my “Utilities” mailbox using Mail on my iPhone. Later I went looking for these two receipts on my Mac … and they weren’t there. They weren’t in the inbox either. In fact, a search of my entire mail account on my Mac turned up receipts only from last month and before.
I refreshed Mail on my iPhone and it wouldn’t load a preview for either message. If I tapped a message, the first one wouldn’t load at all, and just displayed a throbber. The second message, if tapped, would appear after some delay though. The two emails in question are at the top of this screenshot.
After not finding the messages in Mail on my Mac, I checked iCloud Mail. The emails weren’t there either, so clearly they solely existed—and that only in part—on my iPhone. My Internet provider, Nucleus, was happy to re-send my receipts, so I decided to just delete the corrupted email and move on. But these two stubborn messages refused to delete as well.
The long search
I posted to Apple Support Communities hoping a guru would know the answer to my mystery messages. Nobody replied, however, so I went sleuthing on the Internet once more. As I searched the Internet for a solution, I tried to reset my iPhone’s memory of any traces of mail, in order to rid myself of these bad emails. I turned off iCloud Mail on my iPhone … and Notes, Reminders, and Calendar (since some of these services used to share some syncing technology, and still might). I then checked Settings > General > Storage and iCloud Usage, and found that Mail was still eating up 250 MB of space. I then signed out of iCloud and deleted my Gmail account to make sure everything was gone. And Settings still reported that Mail was taking up a quarter gigabyte. Clearly my mail had not been erased even after turning off and deleting every mail-related account on the device.
Getting concerned, I was hovering my finger over “Erase All Content and Settings” nervously, considering the implications, when I decided to have one more crack at the Internet. Down in the depths of a comment section, a helpful Troy Parry had discovered a solution! Skeptical, knowing that system apps on iOS can’t be deleted, I held an icon on the Home screen until they started wiggling—and to my surprise the small “X” appeared on the Mail app, allowing me to delete the entire Mail app from my phone.
There you have it, folks: to erase all traces of Mail to start fresh when things are misbehaving (or you just want to reclaim the space occupied by mail attachments and things), on iOS 10 or later, just go delete the Mail app. Then redownload the app via the App Store. It’s the least obvious route to accomplish the necessary task of resetting Mail to its original state.