The open-source and cross-platform e-mail client Postbox rolls out another beta and has been quickly adding muscle to its abilities.

Postbox beta 10 introduces an HTML signature-creator in the Compose window. (Credit: Screenshot by Seth Rosenblatt/CNET)

Based on Mozilla Thunderbird and currently available for Windows and Mac, Postbox beta 10--the third update since I checked out the program for in the beginning of February--introduces several small changes worth noting.

Since then, Postbox has seen Hotmail support, Flickr integration in the Compose window, and a host of bug-fixes including two Firefox security updates. In the most recent version, users get the ability to create and edit HTML signatures from the Compose Sidebar's Signature panel and improved calendar attachment detection.

There's also a fix for the spell checker, which had been marking contractions as misspelled words, and a new feature that prevents a message from being marked as read until it's been viewed in the preview pane for a specific minimum time.

One feature that I've just noticed is that the Message pane lets you highlight text, which you can then drag into the search bar. Once you release it, Postbox will open up a search results page in your browser for the highlighted term.

These features continue to improve on the Postbox experience. Without support for extensions and the Thunderbird calendar extension Lightning, though, and keeping in mind Postbox's social-networking friendliness, it'd be interesting to see baked-in support for a Web-based calendar as an alternative to this major and missing feature.