It's not, though - it's about blackholing unknown sources. Yes, this makes it incredibly difficult to self-host or to start up a new provider in the space. But from the perspective of anyone trying to protect their users, it makes sense.
By definition, every new source has never sent spam -- but it's reasonable to assume that an unknown source is likely spam, however unfortunate that may be.
By definition, every new source has never sent spam -- but it's reasonable to assume that an unknown source is likely spam, however unfortunate that may be.