Try delivering to invalid email addresses too many times (too many of course being up to each mail provider), and you will be the one shitlisted (and rightfully

Try delivering to invalid email addresses too many times (too many of course being up to each mail provider), and you will be the one shitlisted (and rightfully so, as you are likely bruteforce enumerating valid email addresses). For any small provider, getting on the shitlist is catastrophic as unlike the big providers, getting off of it will be hard / impossible.