Using Mozilla's own code, Postbox rivals Thunderbird for alternative desktop e-mail clients. Or at least, it wants to. Make no mistake, Postbox introduces some innovations that Mozilla could learn from. Much as Songbird changes Firefox's tune for audiophiles, Postbox takes desktop e-mail hard toward Web 2.0, with fast links to upload contacts to Facebook and pictures to Picasa.
Click on an e-mail, and the preview pane not only shows the text, but extracts all links, images, other attachments, and contacts into a Compose Sidebar for easy management. From there, you can upload to Twitter, FriendFeed, MySpace, Delicious, and Google. Postbox is also obsessed with tabs, so that each new mail can open traditionally or in a tab of its own. Searches get their own tabs, too. Postbox will ask you to import e-mail, contacts, and other messaging data from Thunderbird, Outlook, Google, and Yahoo when it starts. Two other innovations are worth highlighting: you can globally extract attachments, links, images, or contacts into a single tab, and there's a remix of Thunderbird's Labels feature. Called Topics, it automatically searches across folders for messages with the same tag. This can't be done without extra effort in Thunderbird.
Postbox gained Thunderbird's extensibility in beta 13, but it's currently limited. There's heavy customization is limited, although Postbox does now have it's own slightly-buggy version of Thunderbird's Lightning calendar plug-in. There's also support for the Provider for Google Calendar plug-in. All the add-on talk may be a major interest for some, or much ado about nothing, but the beta won't be free for much longer: Postbox plans to go premium in the fall of 2009.