How can I “Delete All” from Inbox mail folder? [duplicate]

This was the only way at the time in iOS 8 - for newer iOS [possibly 13 onwards but unconfirmed], see iPhone/iPad Mail - rapid mass delete

• If it is not already showing, from your Mailboxes view, select Edit & check the All Trash option. This will then be added to the bottom of your Mailboxes list. Needed later.

• From All Inboxes, or your chosen Inbox, select Edit, then Mark, Mark all as read [seems to make a difference, toggle unread/read just so it gets the idea].

• Then Edit, check the first item, which enables the Mark, Move & Trash options.

• Hold Move & keeping it held, uncheck the item you just checked.

• Release Move.

• The Move options screen should appear.
  Select Trash.

• Wait until all items have been moved [the graphics seems to do it immediately, but experimentation has shown it can take a while to actually move, if there are a lot.

• Return to Mailboxes, select All Trash.

• Edit, Delete All, done.

Caveat... It seems that if you open any individual Mailbox after doing this, they are all returned. I think you may need to wait for the Settings > Mail... > [mail account] > Advanced > Deleted Messages, Remove... period to expire, before it becomes permanent.