Yahoo! is tightening up their spam controls with a significant new defense system. They are now rejecting e-mail coming from zombie computers and identifying them with information from ISPs and third-party companies. When unauthorized computers attempt a connection to the Yahoo! Mail back-end mail servers, they’ll be informed that their unsolicited mail is not welcome.
For example, users with a dynamic IP address will no longer be able to send mail directly to Yahoo! servers, and must instead use their ISP’s designated mail gateway.
Users interacting through the web interface or the Yahoo! Mail POP/SMTP/IMAP services will not be affected blocked by this change. Aside from the spammers, this change will only impact people running a secret, unauthorized mail server in their basement.