There’s another new feature out in Gmail with the “old” compose box. (I have yet to see it with the new compose box, but from the looks of it, they have a number of “coming soon” features to add to the new compose box and I hope/imagine this will be one of them.)
If you use folders and subfolders for organizing your email, you can now add the labels before you send your emails. In the compose box, you’ll see (as shown above), the “Labels” button. Just click, scroll and find the label or folder that you want this sent email and subsequent email conversations. Add it and send your email.
Another great new feature for keeping your email organized!