allow memory hole headers as anti-replay mechanism if user omits x-listname
memory hole provides signatures over relevant headers. if a cryptographically-signed To: header includes foo-request@example.org, then there is no need to require the message to contain x-listname to defend against replay attack (as noted in #158 (closed)). memory hole is more convenient (for MUAs that already implement it), so it would be a usability improvement to accept it as a legit anti-replay mechanism.