Couldn't this functionality be implemented at the level of the MTA, for example by a milter that autoresponds with payment instructions to any sender who attemp

Couldn't this functionality be implemented at the level of the MTA, for example by a milter that autoresponds with payment instructions to any sender who attempts to contact the recipient without having paid? Then users wouldn't have to look at your web page at all and you wouldn't have to apologize to HN readers for its appearance. An MTA level solution also avoids the obvious potential problem of creating an aftermarket for high value email addresses. Enterprising brokers could acquire them initially by paying the money to solicit a reply, and then they could recover their investment by selling the email address at a discount to whoever else wants to contact the recipient.