Dropbox has started giving users an additional 1GB of free storage when they link their Mailbox account into the service via the iOS Dropbox application. Users must simply install the Mailbox for iOS application, head to the settings menu, and then tap the "Dropbox" button. After entering your information, the accounts will sync and 1GB of extra space will appear on your Dropbox account.
If you're already a Mailbox user who has previously connected to Dropbox, all you have to do is unlink them and reconnect again and the additional 1GB of storage will be added to your account. Unfortunately, this only works for users who own an iPhone or iPad as the Mailbox app is not available for Android.