If I were to do this, I'd just remove the "instagram." bit, and just map [1 set of random letters] -> "Instagram" somewhere.

I wonder if companies selling their users' email addresses don't already filter out such personalized addresses anyway, so e.g. Instagram would filter out email addresses out containing "instagram", "ig" or "gram" before selling the data (or to corporate-speak it, "share it with third parties to enhance our customers' experience").