I have also received it and it usually is sent once a month from what I have seen,it is just a statement for interest,even when you don't have the minimum amount with Satoshis to give you interest daily this email tells the statement also about the total interest earned through all of your time in Freebitco.in website.It is nothing fancy and I believe everyone who have received interest at least once in the website,I think gets this email once a month,most likely this is geared more toward the Premium Members rather than free ones so the free members may not receive it.