I once implemented a similar system on a Mercury mailserver and PegasusMail mailreader. Not so sophisticated as you are planning, but similar in idea: a "human" must send back a response to the challenge issued by the mailserver before mail was allowed through.

The amount of abuse this has generated was appaling. Few of the people who mailed me even read my friendly message asking them to send an email to a certain address to verify they were not spammers. Most of them took heavy umbrage of their precious emails not being allowed in without further ado. I finally discarded this system for a classic automated filtering approach.

So be prepared that few users will appreciate your efforts.