[jiggy2011]

The problem with that of course is that automated emails won't get through unless they can also solve your CAPTCHA or you have whitelisted them in advance which would either make usability worse (when you sign up for a service you have to remember to go and whitelist them) or your CAPTCHA has to easy enough for a bot to break which basically defeats the point.

[Rudism]

Agreed. You would have to pre-whitelist, or at least have a junk folder you could look in where non-captchad messages get stored for a certain amount of time before being purged so that you could fish out the emails from non-whitelisted addresses that you're actually expecting to get (and then presumably whitelist them from there for future emails).

[jiggy2011]

The problem with that is that you're actually going to look at email in your junk folder. Spammers would love this because they know that there's a chance that you'll actually see their crap even if it gets filtered.

The entire point of having a junk folder is you don't have to check it apart from the rare occasion you get a phone call saying "hey , I sent you an important email why haven't you replied?"