The thumbnail images that you see at the bottom of the screen are your Shelf - this being a multitasking feature of iPadOS15. Each thumbnail represents an open email message, or another open instance of the Mail App. The “grey” icon represents tne currently selected/visible window.
Begin by closing each of the other windows. When the shelf is visible, to close a window, simply swipe the corresponding thumbnail off the top of the screen. When all have been closed, perform a forced restart of your iPad:
- On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
- On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.
This will hopefully resolve your current difficulties.
For information, you can learn more about iPadOS multitasking and the shelf here:
https://appleinsider.com/articles/21/08/12/how-to-master-multitasking-on-ipad-and-ipad-pro-in-ipados-15
The iPad User Guide is also a rich source of frequently overlooked information. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:
iPad User Guide - Apple Support
https://books.apple.com/book/id1567104892
