Developer Postbox says it has opened up beta testing of its self-named e-mail software, which can now be downloaded by the public. The app said to differ from most e-mail clients by cataloging all content within a user's e-mail, including links and attachments. This enables a user to not only search for general keywords but filter by categories, such as imagery or documents.

Messages themselves can be organized by topic, allowing users to filter out unrelated material. The scheme is also used to create distinct responses, as well as annotations and to-do lists. To-do lists are typically generated by marking messages with the appropriate tag, which bumps e-mail to the top of the inbox; new to-do items can also be created wholesale.

Both the Mac and PC and versions of Postbox are currently free downloads.