greytrapping: they send to junk addresses
Pioneered by Bob Beck and the OpenBSD spamd team
Create a list of bogus addresses
Any IP address which sends mail to known bogus addresses get blacklisted for 24 hours
Blacklisted hosts get e-x-t-r-e-m-e-l-y s-l-o-w SMTP responses
Shifts the load back to the sender