If the app has verification tools then this might be legit mail. I would like to know if any other users are getting the same mail after they ask the support to unfreeze their account.
Looks like one of the members too said that he was told to download the app.
If you are to download the APP, I suggest you download directly using the app links that appear on their official website and not the link sent to you through the email just to be safe, log in and see what next they want you to do. I hope you did not keep a large amount of funds in Poloniex.