This is a legitimate Dropbox communication. The sender is “dropboxmail.com”, which is unusual but legitimate, and the URL is a secure link (https) to “dropbox.com”.
Your Dropbox is full and is no longer syncing files. New files added to your Dropbox folder won’t be accessible on your other devices and won’t be backed up online.
Upgrade your Dropbox today and get 1 TB (1,000 GB) of space and powerful sharing features.