More helpfully, salting their db with real emails but fake contact info requires a more durable hygiene process and often isn't worth the effort for data driven shops.
You serve a variety of email domains that validate as deliverable, then you accept emails and report the sender, which hurts their deliverability.
My guess was they are referring to one of these services that check the validity of any email address. A false signal from one of these services prevented me from signing up for some random website with a .name domain the other day.
