It would appear that both Mailbox and Carousel have not been nearly popular enough for Dropbox to continue running the lot. The one is used for email, the other for photos. The both of these apps and ecosystems have not found enough users to remain feasible to continue running. Dropbox has decided that the features that people love best will be rolled into products that remain viable – like Dropbox and the lesser-known Paper.
Key features from Carousel will be pushed back into Dropbox. There the services will become one – your photos are there anyway, right? Bits and pieces of Mailbox will be integrated with Paper, and “new ways to communicate and collaborate” will be integrated in to Dropbox.
Mailbox will be shut down for good starting on the 26th of February, 2016. After that time, your mail will effectively be gone – unless of course you’re using a different server, which you should very likely have been doing anyway.
Carousel will be shut down on March 31st, 2016. This should give you additional time to release the photos from their bonds on Dropbox servers – you’ll likely just transfer everything over to your standard Dropbox one way or the other. Or do some heavy downloading.
If you need additional information about the shutdown, be sure to hit up Dropbox proper.