Agreed. Excessively unfortunate that they didn't send it from @dropbox.com, as I'd never heard of dropboxmail.com.
Isn't it ridiculously easy to spoof the "from" address anyway?
You should pay a lot more attention to the where the links go than where the email is from.
What about emails that are sent from services like Campaign Monitor and the like? There's almost no way to know where that link's going to end up because of the 'tracking middleman' that they all have setup.