I've been doing some mailing list analysis recently (in Ruby), and would be very interested in porting it over to Smalltalk. (I was actually getting really frustrated at the lack of proper debugging setup in Ruby, even though it had some great mail-related libraries). I was looking at thread lengths, numbers of unanswered threads, etc.
Alexandre -- I haven't been able to find a good Mail parsing library for Smalltalk (preferably one that reads the Mbox format natively), I'd be curious to know what you end up using.
As for the download URL -- the link Marcus gave is, unfortunately, in Piper-mail's own format (a simplified version of mbox, really).