Peter N. M. Hansteenhm. this thing seems to be popping up again and again https://news.ycombinator.com/item?id=47636937 #greytrapping #email #runyourownmailserver #spam #spamd #greylisting #antispam
It might be worth mentioning that one commenter on the hackernews item feels that this https://nxdomain.no/~peter/they_do_not_understand_mail_at_all.html makes the point better and is an easier read.
For those with incrementally broader interests in networking and related trickery, https://nxdomain.no/~peter/its_real_its_here.html may be good news (and according to some, the referenced tome can be a good read).