How about sending "cookies" for first-mailers into a NetBSD-list? Unless
you reply (!) the cookie to majordomo, your mail will be delayed and not
automatically posted. Once you have send the cookie back, your mail-address
is stored as "possible human user" for further use and your first mail
and future mails are then beeing posted as ever.

 Spammers usually do not reply, do they? This way the list is as open as
before, but as much closed to spammers as possible. Some small unconveniency
at the start, but this is a fair price.

 Needs reprogramming of majordomo / petidomo of course.

-- 
Markus Illenseer