A tweet today from Zack Shapiro seems to indicate that not only has Mailbox been abandoned by Dropbox with the whole team re-assigned to other projects, but that it's been abandoned for a few months now:
It’s sad to hear @mailbox has been abandoned inside of Dropbox. I’d love to buy it from them and keep working on it.
It's unfortunate but not surprising. Mailbox hasn't had any updates in months, the Mailbox Blog hasn't been updated since April. Mailbox was one of the first new style of email application that really tried to challenge the way we think about email. It introduced a lot of interesting concepts that have been copied in more recent apps such as Spark.
Mailbox had a very devout following and will be missed by many.