I recently heard from Sherman Dickman at Postbox. They're building a very interesting mail client, implementing a lot of the tools I think will be essential for working more effectively with your email. Their focus on tagging, search and organization is spot-on, the web has raised the bar for interacting with large data sets. Why can you search Google in 0.02 seconds, but your mail can take minutes? They have some strong tools for quickly previewing all the content in attachments too, another big opportunity that conventional clients are missing.
They're implementing a Mac client initially, which might be a smart move considering their main competition in the professional market is the sadly-neglected Entourage. It's not released yet, but you can learn a bit more from this TechCrunch coverage.
I'm really pleased to see them moving forward with some innovative solutions, and look forward to downloading it once there's a version available.