Quite some time ago now, I tried sup but found it's indexing latencies unacceptable for my workflow. I also found the user interface a bit foreign and hard to get into.
More recently I've found notmuch, a project that started as a C rewrite of the core bits of sup. Basically, it's a program that indexes and searches through your existing mail.
I had two issues with it.
- it had an emacs interface, and
- it uses maildir instead of mailbox.