Accuracy - Yahoo, Grey Listing etc

What's going on with accuracy?

Yahoo

Email Checker works by talking to email servers to check if a email addresses are valid. To do this, it uses a "standard" conversation protocol to "talk" to email servers. Yahoo mail servers use a non-standard sequence of "conversation" which makes it impossible for us to validate email addresses.

At this time, there is no known work around for this problem with Yahoo servers.In the next few days, we're going to flag all Yahoo email addresses as "Blocked/rejected" because there is simply no way to check email validity against their servers.

Grey Listing

The good news is that we'll be including features to handle mail server "Grey Listing". This will make validations much more accurate. As far as we are aware, no other email address checking products can handle grey listing mail servers.