To delete a group of files at one time from IOS Mail:.
- In the Mail folder from which you will make the deletes, select "Edit".
- Specify your target in the "Search" box.
- Select the first item.
- Press and hold "move".
- Deselect (tap) the first item and wait for Mail to create a stack of items in the read column.
- Release "Move".
- Select from the list of actions.
This process will allow any of several listed actions on items from one or more folders.