Greylisting is an anti-spam technology deployed by mail servers to prevent spam. The receiving mail server tells the sender, to please retry to send the email again in X minutes.

A well configured mail server will attempt to resend an email on different intervals, until it’s accepted or rejected by the receiving mail server.

There are many types of greylisting, but the most common is time-based greylisting as mentioned above. Each mail server has a different time they greylist the sender for, per email, normally between 1 and 15 minutes.

Zerobounce deploys break-through anti grey-listing technology. When greylisting is encountered we will wait 15 minutes and retry to validate those specific emails. This reduces the amount of “Unknowns” you will receive back from us. So you wait a little longer for your results, but your results will be a lot better.

It doesn’t matter how many greylisted email addresses you upload to be validated, whether it’s 1 or 100,000. The processing is only delayed by 15 minutes in total.