How do I delete all emails on the iPad 2 all at once?
Except for an account's Trash mailbox, there is no such option.
Habitual whiners would likely have a complete mental breakdown if they accidentally selected delete all for an account's Inbox mailbox or sent mailbox.
You can select each email you want to delete in edit in the top corner and then can delete all selected at once in bottom corner. Good luck.
Tap the rot button at the top of the email list, then tap the circles next to the emails to select them once you have selected the emails you want to delete tap on the red Delete button at the bottom to delete them.
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