“…In C/R systems, senders not on the whitelist are required to respond to an automatically generated reply to prove their validity, or to carry out some computation that would cost a spammer nontrivial efforts to deal with its challenges (Denny et al, 2006) (see also the economic solutions in Section 2.3). Care must be taken to the use of C/R systems, for otherwise they could lead to, e.g., deadlock, denial of service to an innocent third party, and excessive burden to mailing list (Templeton;Self, 2004).…”