Postbox, the email client built for heavy email users, entered Public Beta on Sunday. Postbox is designed to allow you to search, organize, and manage your email more more efficiently.
Postbox includes a powerful search tool that lets you use Gmail-style search terms (like "from:Steve" or "before August 2008") to easily find messages. The Gmail-like features don't end there: Postbox automatically organizes threaded emails by subject. Messages can be tagged with one or more topics for easy recall later.
Perhaps Postbox's greatest feature is its ability to recognize what's in your email, and keep it front-and-center. For instance, if you're composing a new message, you can use the Compose sidebar to find attachments in other messages to drag into your new message. The inspector panel on the preview window highlights links, addresses and attachments in the message so you can get to them easily.
For me, I use my inbox like a to-do list. Thankfully, Postbox has a built-in Archive utility that lets you specify an archive folder for each account (which I already had set up). Once you're done with a message, clicking Archive or pressing A moves that message (or many messages) right into your archive.
Add to this to-dos, integrated search, social network integration, message annotation, tabbed browsing, and a high-security Mozilla foundation, and you've got an amazing Swiss Army-knife tool for hard-core emailers.