|No such thing as a small change
I think statistical junk mail filters like DSPAM and SPF are great, and better than nothing, but I can't stand the false positives. Losing any legitimate mail is not acceptable - and the reason most people have a junk email folder, and not simply delete it automatically.
Segmail specifically will never have a false positive by design. No legitimate mail will ever get filtered. This is a big win, because it means you can delete stuff marked as junk straight away.
As for the "same old problems inherit in unintelligent filtering and/or challenge/response systems", if you could state what those same old problems are - perhaps I could address them. I think you may be wrong, and many seem to agree with me.
No offense, but it looks to me like you have glossed over the spec - and haven't taken the time to understood how Segmail works - before commenting.