Grey listing is an old trick used often on business domains in an attempt to stop spammer. How does it work?
The email server stores a list of IP's matched to destination addresses. It then tells the sending server that the inbox is temporarily unavailable and to try again later. After some amount of time (usually around 10 minutes) the server will respond with the real answer regarding the email delivery.
Ever had a business customer tell you it took 10 minutes to get the email you sent him? Well, his company SMTP server is more than likely using grey listing.
Does this stop spammers? No, absolutely not, it's one of the worst possible attempts to stop spam, and worst of all, it delays most legitimate email from arriving on time unless the sender is whitelisted.
Why does this matter in email validation? Verifying an email on a domain that uses grey listing can take 10+ minutes, as that is how long it takes to make the secondary delivery attempt.
Email Ready makes use of callback technology, which means if we detect a grey listed domain, we retry our validation multiple times until we get a valid respond. We are the first email validation technology on the market to make use of this technique, and it shows our commitment to maximum delivery.