I just received an email from someone I barely knew, to use Dropbox. He had no business sending me this email, and by sending it on his behalf you put yourself

I just received an email from someone I barely knew, to use Dropbox. He had no business sending me this email, and by sending it on his behalf you put yourself and him in this situation where both of you engage in sending SPAM. I stopped short of clicking the "report spam" button, but it cost you some amount of goodwill, so you lose either way. I will be a lot less likely now to genuinely recommend your app to my friends.