Zerobounce deploys break-through anti grey-listing technology. A lot of remote mail servers will issue a 421 response code initially, then after a period of time up to 30 minutes, they will allow the email address to be validated properly.
If any file that you upload to be validated has an email address that we encounter a single 421 response on, it will delay the entire file processing time by up to 35 minutes.
If your file appears to be stuck longer than an hour, please let us know and we'll look into it.
Typically, it takes around 1 to 3 hours to validate 100,000 mixed domain email addresses. So, if your file has 1 million email addresses in it, naturally you would assume it will take between 10 and 30 hours. But, it's not that simple, grey-listing isn't multiplied. So a file of 1 million emails will take between 3 and 23 hours. The average is more like 7 hours.
You next obvious question is: "If someone is validating a million emails addresses, do I have to wait?" The answer is no, because we have multiple servers that process email addresses across different geographical regions.
Typically, there will be little to no wait for your file to start processing. However, it is possible that if a lot of people submit files all around the same time, there will be a delay before your file starts processing. When we start experiencing high queue times, we start adding additional servers to handle the processing. We use all the resources we need to use in order to take care of our customers.