I have become enamored with Unibox's approach to inbox handling over the last couple of years. The key is that it groups by sender regardless of the mailbox and when handling 20+ email accounts, this has proven to be exceptionally handy. It eliminates a lot of extra visual clutter and allows me to quickly mow through many automated messages in ways that no other email app has allowed. There are a couple of issues that keep making me consider going back to Apple Mail.app, but the main thing keeping me with Unibox is this grouping functionality.
Is there a way to modify grouping in Apple Mail.app that would behave like Unibox? So far I've not found any luck in emulating this to such a refined degree, but would love to know if anyone has managed such an organization outside of Unibox.