This may be a stupid question, but are you sure your unsubscribe functionality is working? After "Successfully unsubscribing" to Appsumo (trust me, I saw the picture of the founder when I was done) about 6 times, I was still receiving emails from them, and therefore marked them as spam. I also will mark the companies that don't have an easy unsubscribe all method as spam. (IE, you didn't give me the option to opt-in/out when I signed up, and send me a whole bunch of emails which I have to log in to your app to turn off)