Unfortunately, YouMail will stop development of their Blackberry application. This decision did not come easily, but it was a logical move. According to their blog, because of the decline of Blackberry users and the increase of WP7 users (even without an official WP7 app), they’ve decided to shift focus on their Android and iOS applications.
This was, as they say, “bittersweet” since Blackberry is where it all started for the YouMail team. You can still download the YouMail app in the Blackberry App World with their latest update, but it’ll be the last one for a while.