The "x" on the bottom (left edge) is the "Delete" button.
Be aware that the message will be deleted immediately, since the icon "x" means "Delete immediately" (not: "Move to Trash / Deleted folder").
However, you can change the behavior "how to delete messages" in the settings, for each account separately. Nevertheless the initial setting is “move to deleted first, delete from server when purging the deleted folder”.
To change the default: long press on the account in the main screen and choose “Options and Folders”, then “Account options”.
The icon "x" should than change to a "Trash can".