I believe there are various tools that will do exactly that for you. For instance, I manage my tasks using OmniFocus, and in Apple's Mail program, pressing a k

I believe there are various tools that will do exactly that for you. For instance, I manage my tasks using OmniFocus, and in Apple's Mail program, pressing a keyboard shortcut instantly creates a task in OmniFocus, with the content of the email added as an attachment to the todo, and a link back to the original message (just in case I want to refer to the whole thread or something). Another keyboard shortcut moves the message to my "Old Mail" folder on my IMAP server. (I used to organize old mail by client or project or whatever, but modern searching makes that unnecessary.) I use two separate actions because the normal thing I do with a message is glance at it and put it in Old Mail. If I need to make a task out of it, it's one extra keystroke. So my Inbox only has new messages in it that I haven't seen yet, and my task list has all my tasks, including the ones originating from email, without much friction.