I've seen this method used by some email boxes, but question (not necessarily as an objection) whether the delay to get the reply might have a disconcerting effect on order of postings. Some email services are quite slow in delivering the mail. I've sent myself emails that took nearly a full day to arrive.
With email, your address for the send-back is automatically revealed to the recipient for sending the confirm request (unless you deliberately conceal it) but with a post it would require a separate, specific action for the poster to submit an email addy - with reluctance to do that probably being a main reason why they are GUESTs.
A more common method for posted contributions is just to require a moderator to check them (all posts by GUEST?) before they appear in the thread; but that could put another rather large burden on our elves.
Another fairly common method for killing machine posts is to use an image of randomly(?) generated numbers/letters and require the poster to copy and type them before the post is completed. That could be as automatic as sending an email requesting confirmation. It would eliminate machine-made posts, but not the "just ornery human" ones.