Reader Marcel Sislowitz would like to select a Mail message without opening it. He writes (rather redundantly, as I’ve pretty well parroted what he writes):
In Mac Mail, how do I select an individual message for deletion without actually opening it?
You’ll notice that when the bottom portion of Mail’s viewer window is open, all you have to do is select a message and its status will switch from Unread to Read (denoted by the disappearance of the blue dot in the first column). The trick to selecting individual messages without changing the read status (IOW, without “opening” them) is making the bottom portion of the viewer window disappear.
To do so, simply double-click on the bar that separates the list of messages from the preview viewer below. When you do, the bar moves to the very bottom of the window. Should you wish to see a preview of your messages again, just double-click the now-bottom-dwelling bar and the separator and preview will return.
If the whole point of this exercise is to select non-contiguous messages without opening all of them, just select a message you’re least afraid of (one without an attachment, for example), and Command-click the individual messages you want to delete. Press Delete and they’re gone.