If an email service thinks your email is spam, shouldn't it ask you to solve a CAPTCHA?

There are a number of systems like this in use. I've sent mails to people and it's come back with a link to a URL with a CAPTCHA. I fill in the CAPTCHA and it says it has then delivered my mail to the recipient.

Why doesn't gmail do this?

If an e-mail is spam, the from address is probably forged, so sending a captcha is just spamming someone else.

don't know what you're trying to say, really.

but if you're wanting inboxes to be request verification on incoming mail, it's not going to work very well for the service mails that are truly useful, like bill notifications from your creditors and the like.

and it will inconvenience the hell outta your friends and family...

It would have request verification only when it already thinks your email is spam.