Postbox Express is a lightweight, free application with all the basics of Postbox. It has the very same conversation view to get a better look at an e-mail chain without the baggage of redundant timestamps and indentation. Granular search options allow you to search before and after specific times and dates. Postbox Express also searches each link in every email you receive for phishing scams, checking against a directory of suspicious sites that it updates every thirty minutes. The app also boasts the tight Mac OS X integration and tabbed e-mail features that put Postbox on the map.
Alas, all this comes at the expense of Postbox’s more heavy-duty features. Chief among them, an entire pane that granted the ability to search from a database of all downloads, images, URLs, and contacts; the ability to create and manage to-dos; most of Postbox’s preview modes; and the entire directory of foreign language packs and add-ons. That last one in particular gives Thunderbird 3 a clear advantage in extensibility over Postbox Express.
Fortunately, Postbox Express is free. The only additional cost to you is some minor pestering on the start page to inform you of a few things that the full version has to offer. No Growl popup abuse, no commitments, and no annoying flashing anything.
Postbox Express requires Mac OS X 10.4 or later, and requires at least a G4 or Intel processor and 1 GB of RAM.