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.
To make these features work well, though, Postbox must index your mail. If you have a lot of mail, indexing the mail for the first time does take a little time. I have over 72,000 messages (thanks mostly to TUAW's delightful content management system) and it's about half done after about two hours on my ca. 2006 MacBook. While Postbox indexes, you can't do anything else, but you can stop and restart the indexing process where you left off. After that, though, I hope search functionality returns instant results.
One thing people may not like is the lack of a unified inbox. Each account is separate in Postbox, and shows up in the Accounts pane. Luckily, you can fake a unified inbox by creating a saved search that pulls in messages from any mailbox you like.
I participated in the private beta program, and the software has grown immensely over the last several months. While it's still a little rough around the edges, email users with mostly IMAP accounts (that is, most of their email is organized and stored on the server) can easily give this beta a try without too much commitment.
The Postbox team has not mentioned pricing for the software once it reaches 1.0. The public beta includes integrated Google search, which most likely generates some revenue for the software (much like Firefox and Safari). The beta is free to download and install.
The beta (for both Windows and Mac) is available from Postbox's website, and requires Mac OS X 10.4 or later. For now, Postbox is available only in English, but the team is interested in help with localization.