What or why is this now appearing on my mail messages on my mini iPad?

Having updated to iPadOS15, you have now encountered the “Shelf” - this being a new feature of the now revised iPadOS multitasking environment. 

iPadOS permits you to have multiple “instances” of the same App - each instance being self contained. The shelf, when triggered, shows a thumbnail of each running instance of an App. For the Mail App - different instances/windows might be a different mailbox, open Mail message, or draft. Simply tapping a thumbnail will switch directly to that running instance/window. Each App - such as Mail, Safari, Notes, Calendar, etc. - has its own corresponding shelf.

Individual open instances of an App can be closed directly from the Shelf; just swipe the unwanted thumbnail off the top of the screen. Note however, that Multitasking and associated Shelf functions cannot be disabled.

This article from Appleinsider has a useful explanation of this new feature along with many of the updates to multitasking:

https://appleinsider.com/articles/21/08/12/how-to-master-multitasking-on-ipad-and-ipad-pro-in-ipados-15

Another useful resource:

Use multitasking on your iPad - Apple Support

The iPad User 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

Unlike iPadOS14 and earlier versions, multi-tasking cannot now be disabled from settings.

Apple do invite constructive suggestions and feature requests via their Product Feedback portal. For iPad and iPadOS:

Feedback - iPad - Apple