The icons at the top of the reading pane do not maintain a consistent position. For instance, when viewing a received email that has multiple recipients, the icons from left to right are:
Reply, Reply all, Forward, Mark read/unread, Star, Move, Delete, Mark as spam, More.
The order is the same when viewing a message with a single recipient, except Reply all is missing.
I guess that makes sense but a problem arises when deleting multiple emails:

I want to delete an email with multiple recipients, so I hover over the delete icon and click.
The next email pops up. I want to delete it too and so click again, without moving the mouse.
But the next email has only one recipient so where the delete icon had been, is now the Mark as spam icon. So the message gets marked as spam when it isn't.

This has happened to me more than a few times and I'm sure it's happening to other users as well. When you hover over the delete icon, you expect it to stay where it is and not to have to chase it around the screen!

Please consider fixing this behaviour. Including the Reply all icon for all emails would do the job.