When I select a message in my inbox, what’s the icon in the upper right that looks like a box with a down arrow in it? Tapping it makes the selected message disappear. But it’s not the trash.
If you mean the one to the right of the trash icon, it is Mark Unread. When the message is in an unread state it is replaced with a Mark Read button.
You can long-tap such actions to see what they do (this works in almost all apps). I think the one you mean is the “Archive” button, which moves the message to the folder that is configured as the archive on the settings screen.
Thank you for the “long press” hint to reveal the icon description. Yes, it’s the archive icon. I appreciate your prompt response. Mystery solved.
See also: Managing mail - K-9 Mail