How do I delete mail messages all in one go

It takes two hands

Go to Inbox

Select "Edit"-Button at top right

Select the first email in your folder.

Release that finger.

Press the Move button at bottom and keep it pressed.

While still holding the Move button, deselect the first email using first finger.

Now release both fingers and wait a few seconds

Now Mail should prompt you where to move all of your emails

Select the trash folder

From the trash folder, you can delete all items at once.

Instructions modified H/T to

http://apple.stackexchange.com/questions/138612/how-can-i-delete-all-from-inbox- or-sent-mail-folder-in-ios

https://www.youtube.com/watch?v=fKa-KFjUIGE