"Charge to send email" emails aren't just for humans
With a "stamp" based system only external mail from unknown addresses would be checked, which means that your internal servers and external servers with a known source address wouldn't have to acquire a stamp first. But it would be trivial to modify your own outgoing mail to produce it's own stamp verified on your own authority as well. It'd also help when email addresses are typo'd and accidentally get sent outside the business, (they wouldn't be delivered because of the stamp).
>the alternatives to captchas invade privacy and restrict freedom
Stamps don't, although they do force you to pay a subscription to a verification body.
>In the RW a lot of people and businesses send a lot of bulk e-mail, very legitimate opted-in e-mail.
That could have a stamp attached, if there are loads of complaints to the issuer of the stamps they'd cut you off and you'd have to find another issuer, the issuers would have to police this, because otherwise they'd be worthless.
>No, what's needed is a real ground-up redesign of how e-mail works
You start off with the real world and then suggest a ground up re-write of email?
>Use combo boxes to let user enter the captcha digits
The spammers don't sit there in front of a browser typing out every message in a web form, they have a prog that fills out the form, it doesn't matter whether the text is transferred via drop downs or a text box.
>This says something about the ethics of Newcastle University.
Come on, we all know that security through obscurity is also insecurity through obscurity, by not letting people know that captcha's are easily hacked people would carry on using them unawares.