You can Delete an email using these methods:
- Using the Top Bar Actions in Mail View.
- Using Swipe Options in List View.
- Using the Vertical 3-dot Menu in List View.
You can set to confirm before deleting by tapping More (o o o) | Global Settings | Advanced Features | Confirm Before Deleting.
Note: For Gmail accounts, you can configure the delete action to be Archive instead of Delete using settings.