Re:Grey-trapping (Score 1)

The ISPs are using this approach, it's called a "spamtrap". If you look at the spamcop reports for an IP or SNDS data from Microsoft you can see the number of spamtrap hits. This does not blacklist the sender right away, but it does increase their spamscore.

I think your Yahoo situation is unlikely -- I'm sure Yahoo has some rate limiting/captcha/etc. in place to prevent someone to sign up thousands of accounts programmatically.