San Francisco-based Dropbox has acquired mobile app developer Mailbox, which makes mobile email applications. Financial terms of the deal were not announced, although multiple reports peg the value at around $100M in cash and stock. Mailbox develops an app which helps people sort through their email inbox. Dropbox has been in an acquisitive mood in the last few months. The firm most recently acquired Snapjoy, a photo app developer, and Audiogalaxy, an online music service, in December.