On Mon, Feb 05, 2018 at 05:31:29PM +0000, Brian wrote: > AfAIK. there isn't any way to determine whether a message posted to > -user is from a non-subscriber. I believe some people are using the one of the X-Spam* headers and looking for the LDOSUBSCRIBER substring. Which is extremely non-obvious, and probably not a vector that ordinary users can easily pursue.