One reason Dropbox may have been interested in Mailbox is because people often use Dropbox instead of attaching large files to emails. Gmail recently rolled out a feature that lets users attach files to emails seamlessly using Google Drive, which arguably reduces the usefulness of Dropbox since you have to visit another site to access your files. While there are no signs that Dropbox will announce its own email service, receiving Dropbox attachments inside messages from Mailbox users could be both smart and easy marketing. Or maybe Dropbox is just eager to bring new, design-focused, cloud-centric companies into the fold.