Mark Sapiro writes:
The asking of a question which requires an "obvious to a human but extremely difficult to a machine" answer is probably the best defence as long as the questions and answers aren't fixed over many Mailman installations.
That's a great idea, actually! How about a standard (but optional) feature for the subscription page like this:
*Please read the following description carefully. You will be* *tested on the content later. *
This is the mailman-users list. It is used for communication about the Mailman mailing list management software, primarily among users, although developers often participate. To subscribe Your name: [ ] Your email: [ ]
*Test question: *
Of course the "test question" would be list-owner configured, a la the "security question" for password recovery that is commonly implemented. The portions enclosed in stars would only be generated if the "test question" were non-blank.
The answer to the "test question" could be further obfuscated by placing it on a different page.