Bots spewing spam from ADSL connections used to be the de riguer a few years ago. Increasingly, these are getting defeated because these IP addresses have been identified as being “spammy”, and spammers have moved back to using bots to sign up for free email accounts en masse.
In this case, the attack is via transfer of reputation where the Clean IP reputation of the free e-mail providers are abused.
One way to stop the transfer of reputation, is by revealing the IP address of the captcha solver to the mail recipient. This effectively lifts the veil on a IP hiding behind a proxy, and it is again subject to reputational checks and black lists.
- Clients who use DHCP may end up with an email account that has a poor reputation.
- There may be privacy issues, however, we are talking about a real person sending e-mail here.